Autoimmune diseases are a major public health concern. Autoimmunity results from an inappropriate immune response to one's own tissue and there is evidence that it can be triggered by exposure to exogenous, potentially immunogenic agents. In such cases, these agents activate and select auto-reactive B cells and/or overcome normal immune tolerance mechanisms that keep auto-immunity in check. Idiopathic immune thrombocytopenia (ITP) is an autoimmune disease in which environmental immunogens (viruses, bacteria, drugs and others) have been implicated in pathogenesis. However an understanding of the molecular basis for how exogenous agents select for and induce expansion of self-reactive B cells is limited. Alloantigens expressed on blood cells are usually defined by single amino acid polymorphisms in membrane glycoproteins. Exposure to alloantigens by transfusion or pregnancy can lead to induction of alloantigen- specific alloantibodies. In the alloimmune response, B cells are selected for binding to an epitope consisting of a single antigen-defining, polymorphic amino acid (AA) and other adjacent non-polymorphic AA residues. Following B cell activation, and with T cell help, B cell receptors (BCR) undergo somatic hypermutation, which randomly alters the AA composition of their complementarity-determining regions (CDR). It is likely that this process sometimes alters specific contact amino acids of the CDR in such a way that specificity for the polymorphic AA is lost and binding to non-polymorphic AAs of the target glycoprotein is enhanced, creating an antibody that is auto-reactive. In post transfusion purpura (PTP), a patient becomes profoundly thrombocytopenic following a blood transfusion in association with production of high titer alloantibodies. How an alloantibody that is incapable of recognizing patient platelets causes thrombocytopenia in PTP is unknown. We have evidence that an autoantibody produced concomitantly with the alloantibody is the actual cause of autologous platelet destruction and have developed a mouse model involving cross-strain platelet immunization that mimics PTP. I propose to use this model to characterize the molecular basis for realignment of an immune response from allo- to auto-reactive. Findings made are expected 1) to clarify the pathogenesis of PTP and a similar phenomenon seen in patients producing allo-antibodies specific for RBCs; 2) to lay a groundwork for further studies of human disorders associated with allo-immunity and; 3) to establish a basis for future studies of autoimmunity resulting from immunization to exogenous agents.