Etiology
Autoantibody specificity
• In persons with chronic ITP, approximately 75% of autoantibodies are directed against platelet GPIIb/IIIa or GPIb/IX GP complexes
• Presumably, the remaining 25% are directed against other membrane epitopes, including GPV, GPIa/IIa, or GPIV
Role of the spleen
• The spleen is the site of autoantibody production (white pulp)
• It is also the site of phagocytosis of autoantibody-coated platelets (red pulp)
• The slow passage of platelets through splenic sinusoids with a high local concentration of antibodies and Fc-gamma receptors on splenic macrophages lend to the uniqueness of the spleen as a site of platelet destruction
• Low-affinity macrophage receptors, Fc gamma RIIA, and Fc gamma RIIIA bind immune-complexed IgG and are the key mediators of platelet clearance.
Platelet destruction
• The mononuclear macrophage system of the spleen is responsible for removing platelets in ITP, as demonstrated by the fact that splenectomy results in prompt restoration of normal platelet counts in most patients with ITP.
• Platelets are sequestered and destroyed by mononuclear macrophages, which are neither reticular nor endothelial in origin. Therefore, the former designation of reticuloendothelial system is considered imprecise.
• Immune destruction of immunoglobulin-coated platelets is mediated by macrophage IgG Fc (Fc gamma RI, Fc gamma RII, and Fc gamma RIII) and complement receptors (CR1, CR3).