General Definitions
Platelet. A small blood cell which prevents the blood vessels from leaking. The normal platelet count in our laboratory is 130,000 to 400,000. Platelets are produced in the bone marrow by large cells called megakaryocytes. Examination of the patient's bone marrow is often necessary to make the correct diagnosis in patients with low platelet counts.
Thrombocytopenia. A medical term meaning a reduced number of platelets in the blood. Patients with thrombocytopenia bruise easily (platelet counts less than 30-50,000) and, if the platelets are very low (less than 5-10,000), may bleed from areas such as the nose, gums, stomach, bowel, etc.
Immune system. The body's defense system, which protects it from infection and from foreign substances. This protection may result from the production of antibody or the formation of specialized white blood cells (cytotoxic T lymphocytes).
Antigen. Any substance that the immune system recognizes as foreign (e.g., smallpox or tetanus vaccine, bacteria, viruses, etc.) and to which it responds by producing specialized defense cells or antibodies to neutralize the effects of the antigen.
Antibody. A protein produced by cells of the immune system which binds in a very specific manner to the antigen and causes its destruction. For instance, vaccination with tetanus vaccine results in the production of antibodies against the tetanus bacteria (germ). These antibodies are then available to bind to and cause the destruction of these bacteria should they enter the body, thus preventing infection.
Autoantibody. In some individuals, the immune system makes antibodies against a normal body substance; this can result in an 'autoimmune' disease. Chronic ITP is an autoimmune disease in which the immune system produces autoantibodies that may react with: (1) proteins on the platelet surface and cause platelet destruction or (2) proteins on the megakaryocyte surface and interfere with platelet production. The reasons for autoantibody formation are not known.
Cytotoxic T lymphocytes. Specialized white blood cells that are produced in the bone marrow and then mature in the thymus gland (thus the 'T' lymphocyte designation). Cytotoxic T lymphocytes are programmed to bind to a specific antigen on a cell (e.g., bacteria, cells containing viruses or, in the case of ITP, platelets and possibly megakaryocytes). After binding to the target cell, the cytotoxic T lymphocyte releases toxins that cause the cell to die.
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