The cellular components of blood include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Normal human blood contains 4.8 - 5.2 million erythrocytes/ml and 4000 - 10,000 leukocytes/ml. Leukocytes are divided into five classes based on morphological and tinctorial characteristics when stained. The five classes of leukocytes are:

  • neutrophils (40% - 75%)
  • eosinophils (1% - 6%)
  • basophils (less than 1%)
  • monocytes (2%-10%)
  • lymphocytes (20%-45%)

Collectively, neutrophils, eosinophils, and basophils are known as granulocytes due to the presence of granules in their cytoplasm. In addition, monocytes and lymphocytes are also known as mononuclear cells.


Within 12 hours of being discharged from the marrow into the bloodstream, neutrophils migrate into the extravascular tissue. Tissue neutrophils are activated by chemoattractants at the site of injury. Neutrophils ingest bacteria by phagocytosis and then release enzymes (such as lysozyme) to destroy the bacteria.


Eosinophils migrate from the marrow through the blood into the extravascular tissue, and they survive there for weeks. Again, chemoattractants direct the movement of eosinophils, and like neutrophils, eosinophils are phagocytic. They do not ingest organisms, but they do exert cytotoxic effects on them.


Basophils are morphologically similar to mast cells, and along with other granulocytes, basophils are motile cells with phagocytic properties. They may migrate into extravascular tissues where they may be stimulated by complexes of antigens that are bound to IgE.


Monocytes are larger than other leukocytes, and they mature into macrophages once they are released into the bloodstream. Monocytes then migrate to tissues, particularly the liver, lymph nodes, and lungs, where they may stay for days or years. Here, the monocytes are actively phagocytic, and they ingest particulate matter. Monocytes are also important to the immune response. They ingest and process antigens and are involved in antigen presentation, by B- and T-lymphocytes.


Two main types of lymphocytes are B-cells and T-cells. B-cells are characterized by the presence of immunoglobulins on their surface, and upon stimulation with antigen, they are transformed into plasma cells. Plasma cells are then able to secrete antibodies specific to the antigen. T-cells take part in cell mediated immune response, which does not depend on the presence of circulating antibodies.

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