B lymphocytes use membrane-bound antibodies to recognize a wide variety of antigens, including proteins, polysaccharides, lipids, and small chemicals. These antigens may be expressed on microbial surfaces (e.g., capsular or envelope antigens) or may be in soluble form (e.g., secreted toxins). B cells differentiate in response to antigen and other signals into cells that secrete antibodies . The secreted antibodies enter the circulation and mucosal fluids and bind to the antigens, leading to their neutralization and elimination. The antigen receptors of B cells and the antibodies that are secreted usually recognize antigens in the native conformation, with no requirement for antigen processing or display by a specialized system. Macrophages in lymphatic sinuses and dendritic cells adjacent to follicles may capture antigens that enter lymph nodes and present the antigens, in intact (unprocessed) form, to B lymphocytes in the follicles.

The B cell–rich lymphoid follicles of the lymph nodes and spleen contain a population of cells called follicular dendritic cells (FDCs), whose function is to display antigens to activated B cells. FDCs are not bone-marrow derived or related to the dendritic cells that process and present antigens to T cells. FDCs express receptors that bind antigens coated with antibodies or complement by-products such as C3b and C3d, with no role for MHC molecules. The antigens displayed by FDCs are seen by specific B lymphocytes during humoral immune responses, and they function to select B cells that bind the antigens with high affinity.

Although this chapter has focused on peptide recognition by MHC-restricted CD4+ and CD8+ T cells, there are other, smaller populations of T cells that recognize different types of antigens. Natural killer T cells (called NK-T cells), which are distinct from the natural killer (NK) cells , are specific for lipids displayed by class I–like CD1 molecules. Mucosal associated innate T cells (MAIT cells) are specific for bacterial derived vitamin B metabolites displayed by class I–like MR1 molecules. γδ T cells recognize a wide variety of molecules, some displayed by class I–like molecules and others apparently requiring no specific processing or dis play. The functions of these cells and the significance of their unusual specificities are poorly understood.