Specific and nonspecific mechanisms of action of immunoglobulin G in therapy of idiopathic thrombocytopenic purpura (ITP).

The efficacy of IgG infusion therapy in ITP is now established even in cases resistant to other forms of therapy. However, the mechanism of action is still speculative. We assume that a correction of the elevated thrombocyte clearance is brought about at several levels. First, antibodies specific for an inciting antigen (and for which the patient is deficient) may remove free antigen and/or immune complexes which adhere to platelet surfaces, thereby rendering platelets less susceptible to clearance. Second, IgG may act nonspecifically by protecting the platelet surface from becoming covered with immune complexes. Third, monomeric IgG may display a nonspecific inhibitory effect at the level of the interaction of immunologically altered platelets with Fc receptors of mononuclear phagocytes. For the latter effect, good in vivo evidence exists. However, it must be born in mind that interaction of antibodies with Fc receptors is but one mechanisms for triggering adherence and endocytosis. A variety of other receptors and binding sites exists which may interact with immunologically altered thrombocytes. These may either trigger phagocytes on their own or facilitate the interaction of antibodies and Fc receptors. How IgG infusion influences such interactions remains to be determined.