The initiation of an immune response requires the interaction between T cells, B cells, and antigen-presenting cells (APCs), which form central components of almost all immune responses, and antigens, substances recognized as foreign by the immune system. The ability of an antigen to combine with antibody reflects the property of antigenicity. The distinction between antigenicity and immunogenicity can be seen by examining antigen-antibody reactions; a substance that is antigenic but not immunogenic would likely bind to a B-cell membrane immunoglobulin receptor but fail to provoke subsequent antibody production by that B cell. Researchers have determined the three-dimensional structure of major histocompatibility complex (MHC) class I and class II proteins bound to peptide. This work provided important insights into the nature of T-cell epitopes. To elucidate the differences between antigenicity and immunogenicity, K. Landsteiner, in the 1920s, synthesized numerous small organic compounds that, by themselves, could not induce antibodies but, after coupling or conjugation to a larger molecule, could induce antibodies capable of binding the free compound. Adjuvants enhance immunity, usually by provoking a more intense and prolonged immune response. Common T-cell mitogens include concanavalin A, phytohemagglutinin, and pokeweed mitogen. These mitogens bind surface carbohydrates on cells and may also promote cellular agglutination. Although mitogens are an experimentally useful surrogate for measuring lymphocyte responses to antigens, the results of such experiments must be interpreted with caution since the responses may deviate considerably from the true in vivo situation.