5 Megakaryocytes and platelets in immune thrombocytopenic purpura

In idiopathic thrombocytopenic purpura, 90% of the patients have an elevated platelet-associated immunoglobulin. The most important related antigens are glycoprotein IIb-IIIa (GPIIb-IIIa) (16.7–83.3%), GPIb-IX (13.3–83%), GPIb (3.3–47.1%) and GPIIIa (21.6–33.3%), and less commonly GPIa-IIa, GPIV and GPV. Other related antigens can be platelet granule membrane protein, phospholipid, intraplatelet and cytoplasmic antigens, and rarely human platelet antigen (HPA) la and HLA-DR antigens. The marrow megakaryocytes are usually normal or increased in number with maturation impairment. There are discrepancies regarding megakaryocytopoiesis in vitro. A low dose of heparin could elevate the platelet number in certain cases. The expression of c-sis was reported to be inhibited in ITP, when the plasma β-thromboglobin-platelet factor 4 level was elevated.

In secondary immune thrombocytopenia, platelet antibodies can be (1) alloantibodies against the (HPA) system and (2) autoantibodies most commonly against platelet GPIIb-IIIa or GPIb-IX. Other antigens can be 30–52 kDa proteins of the platelet membrane. Platelet survival is usually shortened, and marrow megakaryocytes are normal in number. Megakaryocyte colony-forming units could be reduced.