I.  INTRODUCTION
 
In the embryo, blood cells first begin developing in the yolk sac.  As the fetus grows, blood cells develop in the marrow of all bones, the liver, and the spleen.  In children, blood cells are produced in the marrow of all bones except those of the face.  In the adult, blood cells are made in the marrow of the calvarium (a bone in the skull), sternum, ribs, vertebrae, pelvis, and hips (the so-called “axial skeleton”).  In adults, the long bones of the arms and legs have only yellow (also referred to as white) marrow, which is largely fatty tissue that does not produce blood cells.  If a patient becomes anemic, this yellow marrow can be converted to blood-producing red marrow.  If the marrow is severely damaged in an adult, “extramedullary” production of blood cells may also occur in the liver and spleen, organs that normally produce blood only in the fetus.  Marrow samples for diagnostic purposes are usually removed from the posterior iliac crest, which is generally a painful procedure.  Samples used to be taken from the sternum, but, if the needle slips, the heart and aorta are a little too close for comfort.
 
The blood is an organ that is distributed throughout the body.  It is composed of formed elements (red blood cells, white blood cells, and platelets) and plasma. The term plasma refers to the liquid that is left when anticoagulated blood is centrifuged to remove the cells.  The term serum refers to the liquid that is left after the blood clots and the clots are removed.  A very important point is that serum does not contain coagulation factors and cannot be used to perform coagulation tests.  Plasma contains coagulation factors and can be used for clinical coagulation tests. A blood smear allows the examination of blood cells for diagnostic purposes. One can even determine the gender of a patient from a blood smear. In women, one of the two X chromosomes in each cell is inactivated and condensed.  This inactive chromosome forms a small, darkly staining body (called a Barr body) that can be seen in the nucleus of neutrophils.
 
Plasma is obtained by collecting blood in tubes containing an anticoagulant.  After this blood is centrifuged, the red blood cells (RBCs) are located on the bottom (they are heaviest).  The percentage of the total volume of the blood that is taken up by these packed RBCs is referred to as the hematocrit.  A normal hematocrit is about 40 to 45 (meaning that 40 to 45% of the volume of blood consists of RBCs), although this can increase under conditions where there is high demand for oxygen (e.g., in people who live at high altitude).  The RBC layer is topped by a cream-colored buffy coat, which contains white blood cells (WBCs) and platelets.  The plasma is located above the buffy coat.
 
The appearance of the blood can be useful diagnostically.  The plasma is normally pale yellow, due to pigments from hemoglobin and other iron-containing proteins.  A lower than normal hematocrit indicates anemia.

• A low hematocrit and very pale plasma indicate severe iron deficiency anemia.  The plasma is pale due to lack of iron-containing pigments, and there are too few RBCs since iron is required for their production.
• A very large buffy coat and a mild anemia may indicate leukemia.
• Intensely yellow-colored plasma may indicate jaundice.  The color is due to excessive bilirubin, a product synthesized from hemoglobin.  This condition may result when the liver is not working properly, since it is the liver that processes bilirubin to facilitate its excretion.  Another possible cause of jaundice is hemolytic anemia, when the RBCs are being broken down at an excessive rate in the circulation.  The liver may be working well in this instance, but the amount of hemoglobin being released is too high for it to handle.  (More to come on this topic in the liver lecture.)
• An increased hematocrit (polycythemia) can result from a neoplastic condition called polycythemia vera or could occur in any oxygen-deprived patient.  There may be an increased tendency to clot, since the blood doesn’t flow as well when the hematocrit is greatly increased.

Blood also contains salts, of which sodium and chloride are the most abundant.  (Potassium, in contrast, is located mostly within cells.)  Plasma contains many proteins; the most prevalent are albumin and gamma globulins.  Plasma also contains and transports carbohydrates, lipids, and gases.
 
 
II.  ANEMIA
 
RBCs transport oxygen, which binds to the hemoglobin molecules that the red cells contain.  Hemoglobin molecules contain heme groups, which consist of four pyrrole rings held together by ferrous iron.  Synthesis of hemoglobin requires vitamin B12, vitamin B6, folic acid, iron, and adequate nutrients to make the protein part of the molecule (globin).  When RBCs die, the heme gives rise to bilirubin in the spleen.  RBCs can change their shape (deform) if necessary, but their normal, unstressed shape is determined by the hemoglobin.  The “biconcave disk” shape of the RBCs allows for efficient exchange of gases and also allows the cells to deform easily, e.g., if they need to squeeze through a capillary.  RBCs circulate in the blood on average for 120 days, making them the longest-lived of all blood cells. 
 
Usually problems of the blood are the result of having too few cells or too many cells, but sometimes disease is due to disordered function or an abnormal immune response.  99% of the time, diseases involving RBCs result from having too few cells (anemia).  In diseases involving WBCs, having either too few or too many cells can cause disease.
 
A.  Values to assess the nature of RBCs
 
Anemias are categorized by their cause and by changes in the following indexes: 

• Red cell count:  the number of RBCs per unit volume of blood
• Hemoglobin:  the amount of hemoglobin present in the blood, expressed as grams
• Hematocrit:  the volume of blood that is occupied by RBCs, expressed as a percentage
• Mean corpuscular volume:  the average volume of an RBC; indicates whether RBCs are larger or smaller than normal
• Mean corpuscular hemoglobin:  the average amount of hemoglobin in an RBC
• Mean corpuscular hemoglobin concentration:  the average concentration of hemoglobin in an RBC, which is obtained by dividing the hemoglobin by the hematocrit. This value indicates whether the amount of hemoglobin is appropriate for the size of the RBCs.

The following terms are used to describe the shape and color of RBCs.  Recall that normal RBCs are disk-shaped, with a concave center.

• Normocytic:  the size of the RBCs is normal
• Microcytic:  the RBCs are smaller than normal
• Macrocytic:  the RBCs are larger than normal
• Normochromic:  the color of the RBCs is normal, indicating that the hemoglobin levels are normal
• Hypochromic:  the RBCs are paler than normal, indicating that the hemoglobin levels are low

Reticulocytes are RBCs that are released before they are fully mature.  A mature RBC has no nucleus, but a reticulocyte has nuclear remnants.  The measurement of reticulocytes is important in the treatment of anemia.  When an anemic patient is treated successfully, the body begins to make a large number of RBCs, which may be released from the bone marrow before they are fully mature.  An increase in the number of circulating reticulocytes is an indication of satisfactory treatment of anemia and verifies that the bone marrow is working.
 
As mentioned, RBCs are produced in the bone marrow.  The bone marrow can be pictured as a factory that requires raw materials (protein, iron, vitamin B12, vitamin B6, folic acid), intact space for production, and an external place for RBCs to function (the vessels).  Problems can occur 1) if the raw materials are lacking; 2) if the bone marrow is damaged; or 3) if RBC loss occurs due to bleeding from damaged blood vessels or an excessively rapid turnover of the cells.  Clearance of the RBCs may occur too rapidly if the cells are abnormal in structure, coated with antibody, infected with a parasite, etc., or if the spleen (where clearance takes place) is overactive.
 
In chronic diseases, the number of RBCs tends to decrease.  This anemia occurs because the body cannot devote as much attention as is normal to producing RBCs when it is fighting a disease.  Also, other systems that interact with the blood may be affected by disease.  For example, the kidneys make erythropoietin, a factor that affects RBC production.  In kidney disease, erythropoietin production may be diminished, leading to anemia.  Other blood factors are taken up in the GI tract, so that GI disease may prevent their assimilation.
 
B.  Types of anemias and their causes
 
Anemias can be conveniently classified according to the abnormalities in RBCs that they produce: 

• Macrocytic anemia:  RBCs that are larger than normal are typical of megaloblastic anemia (see below).
•   Normocytic anemia:  an anemia characterized by RBCs of normal size and color may result from acute loss of blood (for example, due to trauma).  The remaining RBCs were produced under normal conditions and so are of normal appearance.  Some chronic diseases also result in normocytic anemia.
• Microcytic and hypochromic:  RBCs that are smaller and paler than normal are characteristic of anemia due to a deficiency of iron.

C.  Symptoms of anemia
 
Anemia can be mild and show no symptoms except in times of stress (e.g., at high altitudes).  It can also affect delivery of oxygen to tissues severely enough to result in dizziness, fainting, lethargy, heart murmurs and/or failure, headaches, shortness of breath, angina, claudication (pain in the muscles of the extremities) and numbness and tingling (due to oxygen deprivation that affects the nervous system).
 
D.     Anemia due to failure of the marrow
Aplastic anemia is due to failure of the bone marrow to function properly.  Leukopenia (fewer than the normal number of white blood cells) is also a consequence. The marrow may fail for many reasons, including developmental problems such as osteopetrosis (see the lecture on the musculoskeletal system) or damage due to radiation, toxic substances, immune reactions, or cancer.  Sometimes the failure is of unknown cause (idiopathic).  Usually, the RBCs in patients with aplastic anemia are normocytic and normochromic.  Treatment options include a bone marrow transplant, if a suitable donor is available.
 
E.     Megaloblastic anemia
 
As noted above, the RBCs in patients with this condition are larger than normal.  This condition is usually due to a deficiency in folic acid, vitamin B12, or both. The deficiency may be due to inadequate dietary intake, especially in vegetarians who eat no eggs or milk.  There is also an extra need for folic acid during pregnancy.  However, inadequate dietary intake is a rare cause in the US.  A more common cause, particularly in an aging population, is the inability to absorb vitamin B12 from the diet.  The mucosal lining of the stomach produces intrinsic factor, which is needed for absorption of dietary vitamin B12. Chronic immune-mediated destruction of the gastric mucosa may result in insufficient production of intrinsic factor, a condition called pernicious anemia.  Many patients with pernicious anemia also have antibodies that block the function of intrinsic factor.  This condition develops slowly, so it has often reached a severe stage by the time that it is recognized. It may affect the spinal cord, causing demyelination, which may result in loss of positional and vibrational sensations.  Patients with pernicious anemia must be treated with injections of vitamin B12, since they obviously cannot absorb it if it is administered orally.
 
F.     Anemia due to iron deficiency
 
Anemia due to inadequate amounts of iron is marked by hypochromic, microcytic RBCs. This is the most common type of anemia in the U.S.  This deficiency often occurs when people diet and do not take iron supplements. It can also occur during menstruation due to the external blood loss, or in pregnancy when the fetus needs iron.  It also occurs due to chronic blood loss, often resulting from ulcers or cancers of the gastrointestinal tract.  A loss of as little as 2 to 4 ml of blood per day is sufficient to cause this anemia.  It is particularly prevalent in young women, due to menstruation combined with a tendency toward inadequate dietary intake of iron.  In underdeveloped countries, parasitic worms (e.g., hookworms) are often a cause of chronic intestinal blood loss.  A sudden loss of blood will not result in microcytic, hypochromic RBCs; the condition takes time to develop.

G.    Anemias due to genetic defects in hemoglobin  

Hemoglobin (Hb) holds and releases oxygen. The normal configuration of the Hb molecule gives the RBC its characteristic biconcave disk shape.  A genetic mutation that alters just one amino acid of the Hb proteins can cause large changes in the shape and function of RBCs.
  1.      Sickle cell disease:  People affected with sickle cell disease have two copies of the gene for sickle cell Hb.  The abnormal Hb gives their RBCs a highly abnormal shape.  These abnormal RBCs are destroyed too quickly by the spleen, leading to anemia.  They are also very fragile and can break, especially when there are changes in pH or oxygen tension.  The abnormal cells can trigger coagulation, causing disseminated intravascular coagulation, and patients can suffer from acute hemolytic crises.  The cells can aggregate and occlude vessels, leading to ischemia and infarction.  Skin ulcers and necrosis of bone may result.  The ischemic episodes can be extremely painful.  The spleen may at first enlarge (splenomegaly) in an attempt to remove the damaged RBCs more quickly, but it may then atrophy due to infarction.  Patients are more susceptible to infection, because they have less oxygen being delivered to the tissues, which in turn impedes the antimicrobial function of neutrophils (remember the lecture on inflammation?).  The spleen also acts to filter out invading microorganisms, and this capability is lost as the spleen atrophies.  People with only one copy of the mutant sickle cell Hb gene have sickle cell trait, which produces much less severe abnormalities.  Sickle cell anemia is most prevalent in Africa and Southeast Asia, perhaps because people with sickle cell trait are thought to be protected from malaria.
  2.      Thalassemia: is also a genetically caused anemia, most commonly seen in people of Mediterranean descent.  It is relatively common here on Long Island, which has a large population of Italian descent.  A Hb molecule in an adult consists of two a protein chains and two b protein chains.  Thalassemia can result from a defect in either type of chain and so can be subclassified as a-thalassemia or b-thalassemia.  The a form of the disease tends to be less severe, because there are more genes (4) for a chains than for b chains (2).  Therefore, one abnormal gene has less effect.  People with only one abnormal copy of a particular Hb gene suffer from thalassemia minor, which is less severe than the thalassemia major that results when a person inherits a defective copy of the gene from both parents.  RBCs from patients with thalassemia have a bump in the center, giving them a “target” appearance, and aremicrocytic.  The RBCs lyse at an abnormally high rate, leading not only to anemia but also to accumulation of iron in the tissues, which can cause a number of severe problems.  The RBCs are cleared prematurely by the spleen, which enlarges.  The RBCs do not tend to jam up in the vessels as in sickle cell disease, so infarction is not a major problem.  
   3.      Spherocytosis:  is also a genetic disease, in which RBCs are spherical rather than biconcave disks.  Again, the abnormal shape leads to impaired function, accelerated removal by the spleen, and anemia.

H.  Other causes of anemia
 
  1.      Parasitic diseases (which are a particularly common cause of anemia in underdeveloped parts of the world):

• Hookworms:  A tiny hookworm larva from the soil can enter the thin skin between the toes and go to the GI tract, where it adheres to the mucosa and sucks blood, resulting in iron deficiency anemia. This used to be common in impoverished areas of the southern U.S., where sanitation was poor and children would often play in bare feet.
• Malaria:  is the most common infectious disease in the world and a major cause of anemia. The malaria parasite invades RBCs and feeds on hemoglobin.
• Babesiosis:  caused by Babesia, which is carried by ticks, and is quite common here on Long Island, where ~60% of the population has been exposed.  Babesia also invades RBCs, but it rarely produces symptoms because the spleen efficiently destroys the infected cells.  It can be a problem in individuals who have had splenectomies and so cannot clear the infection as effectively. Babesia may also cause problems in individuals who have an immune deficiency.  
   

 2.      Mechanical causes:  A certain type of mechanical heart valve consists of a plastic ball in a metal frame.  The ball slams into the frame 60 to 80 times per minute.  This pounding action can lead to significant fragmentation of RBCs, resulting in hemolytic anemia.  Nowadays, porcine valves are more often used, in part to avoid this kind of damage and also because they are not as likely to promote formation of thrombi.  Strands of fibrin can also break apart RBCs.  The fragmented cells reseal, but they don’t carry oxygen efficiently.  Fortunately, this is an uncommon occurrence.
 
   3.      Leukemia-induced anemia: The increased production of WBCs by the bone marrow means that there are not adequate resources for production of RBCs.  A larger-than-normal buffy coat would be seen in samples of blood from such patients.

III.  BLOOD TYPING
 
Blood groups are defined by molecules present on the surface of RBCs. The ABO blood group molecules are sugars attached to lipids on the RBC surface.  Different blood groups have different kinds of sugar groups. Blood group O is missing one sugar; blood groups A and B have a difference in that one extra sugar.  Although RBCs have blood group antigens, they do not have transplantation antigens (MHC or HLA molecules); otherwise, transfusion of blood as presently practiced would not be possible. Cells other than RBCs may also express A or B blood group antigens, which must be considered when transplanting organs.
 
Antigens that resemble those of A and B blood groups are widespread in nature (in plants, bacteria, etc.), so children develop antibodies to blood groups other than their own. People who are of blood group type A will have A molecules on their RBCs and anti-B antibodies in their plasma, and the opposite is true for individuals who are type B.  People who are of blood group type AB will have both A and B molecules on the surface of their RBCs and will have no antibodies to A or B antigens in their plasma. People of blood group type O have neither A or B on the surface of their RBCs but have both anti-A and anti-B antibodies in their plasma.

As a consequence, a person who is type AB can receive RBCs from anyone (universal recipient), since the AB person has no antibodies to any ABO blood group molecules. Conversely, a person who is type O cannot receive RBCs from anyone but another type O individual, since type O people have antibodies in their plasma that would destroy RBCs of types A, B, or AB. Antibodies destroy RBCs by attaching to the blood group molecules and activating complement.  Recall that complement will lyse the RBC or cause it to be removed by macrophages in the spleen. However, a type O person can donate RBCs to individuals of any blood group (universal donor), since these RBCs lack A and B antigens and will not be attacked by anti-A or anti-B antibodies.
 
Note that RBCs are usually administered in the form of washed cells. Any antibodies that are in the plasma of the donor are not an important factor; only the antigens that are on the RBCs to be donated and the antibodies in the plasma of the recipient need to be considered. Of course, the story changes when transfusions of plasma are made:  what blood types would be universal donors or recipients of plasma?
 
The antibodies that are naturally present in the plasma of type A and type B individuals can be used to determine the blood group type of any RBCs.  The RBCs of unknown type are mixed with serum from people of known type A or type B, which contain anti-B and anti-A antibodies, respectively.  If the antigen that is recognized by the antibody is present on the surface of the RBCs that are being tested, the RBCs will agglutinate (clump together).  The blood type of the RBCs can then be determined according to the following chart.  Make sure that you understand the principles underlying this chart!

Important note:  Most transfusion reactions do not result from failure to type and match blood correctly.  Most mishaps arise when the unit of blood and the name of the patient who is to receive it are not double-checked at the bedside.
 
Whenever possible, a cross-matching test should be performed before giving a transfusion, in which serum from a patient is added to the RBCs that he or she is about to receive.  This test checks for incompatibilities between donor and recipient with respect to RBC antigens other than those of the ABO group.  Agglutination of the RBCs in the cross-matching test would indicate such incompatibilities.
 
 
IV.  Rh DISEASE (hemolytic disease of the newborn; erythroblastosis fetalis)
 
RBCs can have not only ABO antigens, but also a so-called D antigen.  Unlike ABO antigens, the D antigen is present only on RBCs.  Individuals who have the D antigen on their RBCs are Rh+, whereas those who lack it are Rh-.  Rh disease has the potential to arise when a mother is Rh- and the fetus that she is carrying is Rh+.  This means that the father of the baby must be Rh+, or else the baby would not be able to inherit the D antigen.  Normally, the blood of the baby and mother are kept separate; the fetal and maternal vessels come close together to interchange nutrients, but not cells.  Therefore, during a first, normal pregnancy, the Rh- mother will not be exposed to the Rh+ cells of the baby, and no immune reaction will be mounted.  However, during the birth process or during miscarriage or therapeutic abortion, blood vessels may be damaged, allowing mixing of fetal and maternal blood.  Fetal RBCs can travel to the spleen, where the mother’s lymphocytes will make IgG antibodies to the foreign D antigen on the baby’s Rh+ RBCs.  These antibodies may cross the placenta and attack the RBCs of Rh+ fetuses in any subsequent pregnancies.  The disease will only affect subsequent pregnancies in which the babies are Rh+.
 
The maternal anti-Rh+ IgG antibodies bind to the RBCs of the fetus, leading to their destruction in the spleen.  The baby may become severely anemic, which can lead to heart failure in utero.  The large amount of hemolysis also leads to excessive bilirubin, which is derived from the released hemoglobin.  Normally, the mature liver conjugates bilirubin, and it is excreted (this gives feces their color).  The immature liver of the fetus cannot conjugate bilirubin, so the bilirubin circulates in the blood and also passes through the underdeveloped blood-brain barrier.  When it enters the brain, bilirubin can accumulate (a condition called kernicterus) and destroy nerve ganglia, causing severe incapacitation or death.  The fetus may also be anemic and develop hydrops (edema) and heart failure.
 
Fortunately, this is a disease that basically can be eliminated with proper prenatal care.  Preventative steps must be taken whenever there is a chance that blood from a potentially Rh+ fetus might mix with that of an Rh- mother.  (Note that the baby’s father must be Rh+ for this situation to occur.)  Within 72 hours following birth, miscarriage, or abortion, such mothers are injected with RhoGAM, which is a preparation of antibodies directed against the Rh D antigen.  Any fetal Rh+ RBCS that might mix with the maternal blood are rapidly coated with RhoGAM and destroyed by the spleen before they can provoke an immune response by the mother.  The RhoGAM antibodies themselves do not persist for long in the mother’s circulation and so do not pose a threat to future pregnancies.  If an Rh- woman receives a mismatched transfusion of Rh+ RBCs, she will develop anti-Rh antibodies.  In this case, RhoGAM will be ineffective in protecting any Rh+ fetuses that she might conceive.
 
Usually, incompatibilities between mother and fetus with respect to ABO blood groups do not cause a problem.  Most antibodies against A and B antigens are of the IgM type, which is too big to readily cross the placenta (see the lecture on immunology).  Remember that the ability of IgG antibodies to cross the placenta is normally a way to provide protection to the newborn.  In Rh disease of the newborn, this normally protective mechanism may go awry to damage the developing baby.
 
 
V.                 POLYCYTHEMIA
 
Polycythemia is marked by excessive numbers of RBCs.  It may be a tumor of the RBCs (primary polycythemia), or it may be secondary to prolonged anoxia, e.g., due to living at high altitudes, chronic lung disease, etc.
 
 
VI.  DISEASES OF WHITE BLOOD CELLS
 
The type of WBC that is present in highest numbers in the circulation is the neutrophil, which matures in the bone marrow.  The primitive neutrophil has a large round nucleus and cytoplasmic granules that are just beginning to appear.  During development, the immature nucleus condenses and assumes a simple oval or “band” shape.  Finally, the neutrophil nucleus becomes segmented.  In females, the nucleus of the neutrophil contains an extra little piece that looks like a drumstick.  This is the inactivated extra X chromosome (Barr body).  Neutrophils, eosinophils, and basophils are collectively referred to as granulocytes.  Granulocytes may also be referred to as myelocytes.
 
During the acute phase reaction of inflammation (remember Dr. Furie’s lecture?), the bone marrow releases increased numbers of neutrophils in response to factors such as interleukin-1.  When release is rapid, immature WBCs (called bands, due to the shape of their nuclei) may appear in the circulation.  Appearance of these bands, as well as cells that may be even less mature, is referred to as a “shift to the left,” because in a standard chart used to tally the types of neutrophils in a blood smear, the column for the less mature cells is on the left-hand side.  An increase in the number of circulating WBCs is called leukocytosis.  In most bacterial infections, the leukocytosis that is characteristically produced is largely due to an increase in neutrophils.  In certain circumstances, the numbers of other cell types increase:

• Basophils are the circulating counterparts to mast cells.  They have large granules that stain blue using standard procedures and that contain many important substances, including histamine.  The number of basophils may increase in patients with atopic (allergic) conditions.
• Eosinophils have granules that stain red.  They, too, are an important source of a variety of mediators.  An increase in the number of circulating eosinophils is common in certain parasitic diseases, where eosinophils release substances that interfere with the reproduction and maturation of the parasites.

Lymphocytes are critical to mounting an immune response (remember??).  The number of circulating lymphocytes may increase in leukemia or in pertussis (whooping cough).
 
Remember that these circulating WBCs can leave the vessels and go into tissues to carry out their various functions (again, see the lecture on inflammation).
 
Some conditions are marked by abnormally low numbers of circulating leukocytes (leukopenia).  For example, HIV infects and eventually destroys CD4+ T helper cells.
 
Cancers of the WBCs are called leukemias or lymphomas.  Leukemias arise from WBC precursors in the bone marrow, and large numbers of malignant cells circulate in the blood.  Lymphomas, in contrast, tend to be located within the lymphoid tissues.  Leukemias and lymphomas are increasing in incidence, perhaps due to environmental factors or to the fact that the population as a whole is aging. In most cases, the etiologies of leukemias and lymphomas are unknown.  However, a few causes have been identified:
·         Viruses:  Epstein-Barr virus, HTLV-1
·         Oncogenes:  see the discussion of the Philadelphia chromosome in CML below.
 
Signs and symptoms of leukemias and lymphomas include anemia, infections, bleeding, enlarged spleen and/or lymph nodes.
 
A.  Leukemias
 
An increase in the number of circulating leukocytes (WBCs), resulting in a large buffy coat and a decrease in RBCs, is characteristic of leukemia. In a “leukemoid reaction,” there may be a greatly increased number of white blood cells, but this reaction is not a true leukemia. True leukemias result when there is an abnormal, uncoordinated proliferation of immature WBCsin the absence of an appropriate stimulus. Leukemias are thus characterized by too many WBCs that are too immature. The leukemic cells may be confined to the bloodstream, or, like lymphomas (see below), they may infiltrate into tissues.
 
There have been great advances in chemotherapy for the treatment of leukemia, particularly in leukemias of childhood. The chronic leukemias tend to occur in older people. Common leukemias arise from lymphocytes or granulocytes (myelocytes). Leukemias are named according to the type of cell from which they arise and whether they produce acute or chronic disease:

• Acute lymphoblastic leukemia (ALL):  is the commonest form of leukemia affecting children under the age of 7 and is also the most curable of the leukemias.  It may also affect the elderly.  It usually has a rapid course, marked by weakness, bleeding, and infection.  It can be treated with chemotherapy, which cures the disease in about 50% of cases.  ALL arising from B lymphocytes has the best prognosis.
• Acute myelogenous leukemia (AML):  is the commonest form of acute leukemia in adults and tends to occur in older individuals, but it can occur at all ages.  It is not as treatable as ALL; the majority of cases are not cured.  Often, bone marrow transplants are used for therapy.
• Chronic lymphocytic leukemia (CLL):  occurs mainly in older people (> 50 years old).  It has been linked to the oncogene bcl-2, which immortalizes lymphocytes.  It is a slowly progressive disease, with a usual survival of 5 to 10 years.  It does respond well to chemotherapy.  Since patients tend to get it late in life and can live with it for a long time, CLL is often not treated at all.  CLL is becoming more common, perhaps due to aging of the population and/or to environmental factors.
• Chronic myelogenous leukemia (CML):  is associated with the “Philadelphia chromosome.”  The Philadelphia chromosome is produced when a piece of genetic information is transferred from chromosome 9 to chromosome 22. This transfer, called a translocation, moves an oncogene to a location where it is no longer under proper control.  The protein produced by the oncogene speeds up proliferation of WBCs in an abnormal way.  This tends to be a disease of adults and is characterized by splenomegaly and a high white cell count. The onset is often difficult to identify. Treatments include bone marrow transplantation.

In addition to genetic abnormalities, viruses may play a role in causing some forms of leukemia.  Epstein-Barr virus infects lymphocytes and causes infectious mononucleosis, which can mimic leukemia.  During mononucleosis, enlarged, abnormal-looking lymphocytes are typically seen within the circulation.  Enlargement of the liver and spleen and fatigue are also characteristic features of this disease.
 
B.  Lymphomas
 
The lymph nodes and spleen act as filters that trap antigens. Lymphocytes within these organs can then mount immune responses to these antigens. Tumors of the lymph nodes or spleen are called lymphomas or lymphosarcomas. They are similar in some ways to leukemias.  There are two main categories:  Hodgkin’s disease and non-Hodgkin’s lymphomas. Non-Hodgkin’s lymphomas can be either follicular or diffuse, depending on whether the tumor cells tend to be segregated in clumps or more evenly distributed throughout the lymphoid tissue.  The tumors can be further classified as low grade, intermediate grade, or high grade, depending on how abnormal the cells appear.  The ability to treat lymphomas depends on the type of cell involved and the stage of disease.  Staging refers to the extent of neoplastic spread and is important for determining prognosis and therapy.  A Stage I lymphoma is confined to a few lymph nodes and can often be treated with radiation.  Stage II tumors have spread to contiguous areas, Stage III have spread to multiple organs, and Stage IV lymphomas are highly disseminated. Staging is not the same as grading, in which the microscopic appearance of the tumor cells is evaluated, as mentioned above.  Staging is usually more predictive of outcome than grading. Staging can be performed by biopsy or by lymphangiogram, in which a substance that localizes to lymph nodes is administered and visualized radiographically.  Abnormalities in nodes that contain lymphoma cells can then be seen.
 
Symptoms of lymphoma tend to be those associated with a hypermetabolic state, as the body puts so much energy into producing tumor cells.  They include fatigue, malaise, weight loss, pruritis (itching), and sweating.
 
There are a number of different types of lymphomas. The most common is the so-called follicular lymphoma, which almost always arises from B lymphocytes. It is of low grade. Diffuse large cell lymphomas can arise from either B or T lymphocytes. These are of intermediate to high grade. Burkitt’s and Burkitt’s-like lymphomas are highly malignant tumors arising from lymphoid stem cells. These malignancies are associated with infection by Epstein-Barr virus and tend to affect children and young adults.
 
It is usually difficult to classify lymphomas based solely on the appearance of the tumor cells.  There is, however, one important exception.  The presence of large, usually binucleated cells with large nucleoli called Reed-Sternberg cells is diagnostic for Hodgkin’s disease.  Hodgkin’s disease is a neoplasm of lymph tissue that can usually be treated with a high degree ofsuccess. There are four different types of this disease. It has a bimodal distribution of incidence with respect to age with peaks from 20 to 30 years old and again from 45 to 55 years old.
 
Lymphomas can involve the bone; for example, it is common to see multiple myeloma in the marrow of the vertebrae and other bones.  Multiple myeloma is a cancer of antibody-producing plasma cells. These tumor cells make large amounts of abnormal immunoglobulin, which can be detected in the plasma.  It is a disease that is increasing in frequency and primarily affects older individuals.  Multiple myeloma is usually a very painful cancer with no good options for successful treatment.  Most tumors in bone come from another organ; primary tumors arising in the bone are relatively rare. 
 
 
VII.  COAGULATION
 
Megakaryocytes are very large cells located only in the bone marrow.  They make platelets, which are the smallest formed elements of the blood and are a key part of the coagulation scheme.
 
Hemostasis refers to the process that prevents us from bleeding to death from every tiny little injury.  There are three factors that contribute to hemostasis.  They are not separate systems, but function as an integrated whole.  The hemostatic mechanism requires the participation of all three of the following:
 
  1.      Blood vessels:  If damaged, vessels will constrict almost instantaneously due to stimulation by nerves.  This constriction will help retard the escape of blood, but it is not always enough to do the whole job.
  2.      Platelets:  When the endothelial lining of vessels is damaged, platelets will stick to the tissue underneath.  The platelets are converted from disks to spiny spheres that aggregate to form a platelet plug within seconds.
  3.      Coagulation proteins:  The slowest part of the system is coagulation.  Coagulation takes place on the surfaces of vessels and platelets and so is confined to where the damage is.  Excessive clotting in inappropriate places is not good! 
 
This process works only in relatively small vessels and under conditions of relatively slow flow of blood.  Do not depend on it to stop arterial bleeding!  (In this case, you must apply pressure or tie off the artery.)  A clot will form when there is a discontinuity as a result of internal (e.g., atherosclerosis) or external (e.g., trauma) factors.  Most disease in this system results from excessive or inappropriate clotting:  it is beneficial to have a clot form in a damaged capillary, but a clot in a coronary artery may occlude the vessel, leading to ischemia and infarction.  There are mechanisms in your body (which can be utilized therapeutically) to dissolve and remove a clot; this process is called clot lysis.
 
Disorders of coagulation can arise from many causes.  Fragile blood vessels may give rise to senile purpura, a disease of the elderly.  Disorders of platelets include autoimmune thrombocytopenia and idiopathic thrombocytopenic purpura (ITP).  Other disorders include hemophilia, disseminated intravascular coagulation, and liver disease.  Liver disease is a particularly common cause of coagulation disorders, because the liver makes many clotting factors.
 
Dysfunctions in hemostasis may lead to two kinds of bleeding, depending on the cause:
 
  1.      Petechial bleeding: The skin, internal organs, or mucous membranes have minute hemorrhages called petechiae, which resemble flea bites.  Petechial bleeding is generally associated with severe abnormalities in platelet function.  If this type of bleeding occurs in the GI tract, large amounts of blood can be lost.  Petechial bleeding can be severe enough to be fatal.  A normal number of platelets in the circulation is 200,000 – 300,000/mm3; generally, abnormal bleeding is not seen until the platelet count drops below 50,000/mm3.
  2.      Ecchymoses and hematomas:  These are large accumulations of blood at one site, often in soft tissues or joints.  These are characteristic of bleeding due to a deficiency in one of the coagulation factors (e.g., hemophilia, which is due to a deficiency in Factor VIII).  The higher up in the cascade that a factor lies, the worse the consequences of its deficiency.  For example, lack of tissue factor or Factor VII is incompatible with life.  Lack of Factor V is a relatively minor problem.
 
 
A.  The coagulation cascade
 
The coagulation system functions as a cascade.  A series of reactions occurs, each one leading to amplification.  For example, one molecule can trigger 10 of another molecule which could then trigger 100 of a third molecule which can trigger 1000 of yet another molecule, and so on....  You can see how the activation of one molecule can lead to a large response.  The complexity of the coagulation cascade also allows it to be regulated at many points; if a molecule is activated inappropriately, it can be inhibited.  Under normal conditions, regulatory mechanisms allow the coagulation cascade to function only when and where it is needed.

THE COAGULATION CASCADE

 
When a vessel is damaged, tissue factor, which is located beneath the endothelium, is exposed.  It interacts with Factor VII, which in turn acts as an enzyme to cleave (split) Factor IX.  This cleavage of Factor IX converts it from an inactive form to an active enzyme.  It, in the presence of Factor VIII, cleaves Factor X.  With the help of Factor V, activated Factor X cleaves prothrombin to form thrombin.  Thrombin converts fibrinogen to fibrin, which is the major constituent of clots.  Most, but not all, of these factors are enzymes.  Some, such as Factor VIII and Factor V, are cofactors that aid in activation of the enzyme factors.  Activated factors are designated by the letter “a” (e.g., Factor Xa).
 
Note that several of the reactions in the diagram above require calcium and “PL”.  PL stands for “phospholipid” or “platelet” (the platelet membrane supplies the phospholipid).  When you see PL, it means that phospholipids, on the surfaces of platelets or perhaps endothelial cells, are involved in that step of the cascade. 
 
B. Clinical tests for coagulation disorders
 
There are many tests that can be used to diagnose coagulation disorders.  Two general tests are:
 
    1.      Bleeding time:  The bleeding time is an in vivo (in the body) test.  It measures the time that it takes for a patient to stop bleeding after a standardized puncture in the skin is made.  It assesses the function of all components of coagulation, including platelets and vessels.  It cannot pinpoint the defect, but it is a good test for general screening.
    2.      Clotting time:  measures how fast a patient’s anticoagulated blood clots in a test tube and is useful for assessing levels of fibrin and platelets.
 
In a patient with petechial bleeding (due to a deficiency of platelet function), either of the two tests listed below may be useful.  (It should be noted that the most common reason for petechial bleeding is not having enough platelets [a condition called thrombocytopenia].)
 
    3. Platelet count:  reveals the number of platelets in the blood, but does not assess whether they are functioning properly.
    4. Clot retraction:  Blood is placed in a glass tube and allowed to clot.  Whether the clot shrinks (retracts) normally is then measured.  Retraction of a clot is due to contractile elements in platelets.  This test can be used as a measure of platelet dysfunction.
 
The fifth and sixth tests screen for bleeding disorders and are often performed as part of a pre-surgical workup.  Usually, both tests are performed.
 
    5. Prothrombin time:  Also called PT or pro time.  This test measures the function of clotting factors that depend on Vitamin K and is therefore good for monitoring the anticoagulant activity of coumadin.  In the PT test, tissue factor is added to blood to initiate clotting.  It does not measure the activity of Factor VIII or Factor V.
    6. Partial thromboplastin time (PTT):  measures all factors except for Factor VII and is therefore a good general test of coagulation.  It can be used to screen for hemophilia.
  
C. Anticoagulants
 
Calcium and vitamin K are essential in permitting certain coagulation factors to bind to PL.  Some coagulation factors require vitamin K for their synthesis. Coagulation factors are made in the liver, so a liver problem may result in a bleeding problem.  If calcium is not present, clotting is prevented. Citrate acts as an anticoagulant because it binds calcium and makes it unavailable for use in the clotting cascade.  Citrate is not used in treating patients, but it is used in collecting blood for testing (blue-topped tubes).  Note that you will never see a patient who is bleeding due to low levels of calcium in the blood, since the heart and muscles would stop functioning and death would occur long before the calcium levels were low enough to affect clotting.  When a patient is given a transfusion of citrated blood, the patient’s own calcium levels are sufficient to overcome any added citrate.  Citrate is a useful anticoagulant for blood that is to be donated, because the citrate can be broken down by the recipient.  EDTA (lavender-topped tubes) is another anticoagulant that binds calcium, but it does so more strongly than does citrate.  Its use is strictly confined to anticoagulating blood in the test tube (and in particular for specimens that will be used for coagulation tests); it is not introduced into patients, even in the setting of transfusions.
 
Other anticoagulants include aspirin, which is very mild in its actions.  Aspirin prevents platelets from aggregating.  Coumadin (also called warfarin) is used as a long-term anticoagulant in patients.  It blocks the action of vitamin K, and so affects all of the clotting factors that require PL and calcium (e.g., Factor IX, Factor X).  It can be taken orally and is rather slow to take effect (on the order of 2 days).  Heparin (which is the anticoagulant in green-topped tubes) enhances the action of a natural inhibitor of coagulation.  It cannot be given by mouth, acts quickly, and has a short half-life.  Due to their different time frames of action, heparin and coumadin are often given to the same patient to ensure rapid but sustained anticoagulation.  ReoPro is an anticoagulant that was developed here at Stony Brook by Dr. Barry Coller.  It is an antibody that reacts with surface proteins on platelets and prevents them from aggregating.  The considerable royalties from the University’s patent on ReoPro have been used to refurbish the HSC library and computing center.
 
The proper flow of blood is very important for prevention of clotting under normal conditions.  For example, if blood pools in the legs due to prolonged inactivity, there is an increased risk of thrombus formation.  (Note:  A thrombus is a clot that forms within a blood vessel and which remains attached to its place of origin.  It is usually a pathological phenomenon.  If a thrombus or a fragment thereof breaks off and travels through the circulation to lodge at another site, it is referred to as an embolus).
 
As you go down the cascade through a series of steps that are mostly enzymatic (one enzyme activates another with the help of cofactors), eventually thrombin is formed from prothrombin.  Thrombin closes the loop, since it activates platelets.  Therefore, both tissue elements (tissue factor) and cascade elements (thrombin) can activate platelets.
 
To summarize, this is the series of events that occurs when you cut yourself:  First, the vessel constricts.  Within seconds, a few platelets recognize and plug gaps in the endothelial lining of the vessel.  Within a few minutes, the coagulation cascade starts.  Thrombin is formed and activates more platelets.  The platelets and coagulation factors work together to stop the bleeding.  Thrombin produces fibrin from fibrinogen.  Fibrin forms the meshwork of the clot.
 
A clot is removed when a plasma protein called plasminogen is converted to an active enzyme called plasmin.  Plasmin breaks down the fibrin in the clot and dissolves it.  Plasminogen can be activated by a protein produced by the body called tissue plasminogen activator (tPA) or by a bacterial protein called streptokinase.  Both of these activators of plasminogen are used therapeutically to dissolve thrombi that are occluding coronary arteries.
 
Sometimes the coagulation system is activated generally, throughout the body, in response to agents such as bacterial enzymes (during sepsis) or amniotic fluid that makes its way into the circulation.  Factors in the venom of certain snakes can produce the same effect.  This generalized activation results in a condition called disseminated intravascular coagulation, where multiple small thrombi form throughout the vascular system.  Formation of these thrombi can use up all available coagulation factors, which can then result in massive bleeding.  This condition is treated by getting rid of whatever is causing the problem (if possible) and by infusing the patient with fresh plasma, but it is associated with high mortality.