Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease wherein the immune system creates antibodies against platelets. Antibodies bind to the platelets’ surface and make them more suitable for destruction. Platelets covered with antibodies are more easily destroyed by the reticular-endothelial system (RES) found in organs like the spleen, liver etc. One of the functions of the RES is to destroy older blood elements (erythrocytes, platelets, etc.). Therefore, we can say that the number of platelets in someone experiencing ITP decreases because of a greater rate of destruction. ITP is diagnosed when an anti-thrombocyte antibody is found in the blood.
In case of Chronic Lymphatic Leukemia (CLL) the number of platelets is also decreasing but because of other reasons. In CLL there is increased production of lymphocytes in the bone marrow where platelets are also produced. The lymphocyte production is so big that suppress the production of the other blood elements (platelets, erythrocytes, granulocytes and monocytes). So we can say that in case of CLL the platelets number is decreasing because of their less production.