The formation of anti-MHC antibody is a significant barrier to improved outcomes in organ transplantation. Patients with pre-formed anti-HLA antibodies have limited options for suitable donors, and the formation of donor-specific anti-HLA antibodies after transplantation is a harbinger of graft rejection. Despite the recognized importance of anti-HLA antibodies, the mechanisms responsible for the differentiation of B cells after exposure to allogeneic antigen are poorly understood. In order to evaluate the differentiation of B cells in response to allogeneic antigen, we used a model of H-2b C57/Bl6 sensitization with H-2d antigen. We found that although the formation of anti-H-2d IgG was robust, few class switched B cells and germinal center B cells were formed. Sensitization induced weak expression of classical memory B cell markers, but we observed populations of CD21+ and IRF4+ B cells, that corresponded to an increase in the frequency of marginal zone phenotype B cells after sensitization. Depletion of marginal zone B cells prior to sensitization resulted in a significant dimunition of anti-H-2d IgG and also fewer germinal center B cells. These results demonstrate a previously unappreciated role for marginal zone B cells as a reservoir of alloreactive B cells that are activated by allogeneic antigen.