One important function of cell-mediated immunity is the recognition of foreign antigens on the surfaces of cells that enter the body. Foreign antigens from these cells are broken down and displayed next to molecules of the major histocompatibility complex of the host's cells. The cells that display foreign antigens on their surfaces are:

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Antigen-presenting cells (primarily macrophages, although some other cells also participate in this function) are the cells of the immune system that display foreign antigens on their surfaces next to molecules of the major histocompatibility complex (MHC) so that these antigens can be recognized as unlike others in the body. In humans, the MHC complex molecules are found on all nucleated cells. They are also known as histocompatibility locus antigens (HLA). When a virus attacks host cells and these cells are consumed by macrophages, the presentation of viral antigens next to the body's own MHC antigens stimulates cell-mediated destruction of virally infected cells. MHC or HLA antigen recognition also is a critical determinant of the rejection of transplanted organs. If a patient receives a transplanted donor kidney, for example, from another person whose HLA type differs substantially from the recipient, a cell-mediated immune response triggered by the recognition of foreign antigens on the donated organ will result in organ rejection. Eosinophils are circulating white blood cells which multiply in allergic reactions. They release histamines and other chemical mediators. Suppressor T cells are T lymphocytes that turn off immune responses. Natural killer cells are lymphocytes without either B or T cell markers that destroy foreign antigens even in the absence of prior exposure to them.

One Important Function of Cell-Mediated Immunity Is the Recognition of Foreign