Stimulation of the cell-surface receptor is the first step in activating each type of cell. Antibodies consist of a light chain and a heavy chain. The light chain is formed when the lymphocyte selects one constant gene region, one variable region, and one joining region from among many. The cell links the components, and the information the components convey is translated to form a specific protein. T cells kill infected cells directly and regulate B cell function.
Antibodies bind directly to intact antigens that are part of larger structures, such as viruses.
T cells bind processed antigen fragments when they are properly presented. MHC proteins play a special role in distinguishing self from nonself. This was first recognized when transplants between individuals of the same MHC type succeeded without immunosuppression of the recipient. Each individual has a distinctive set of MHC genes that code for the synthesis of MHC proteins. Some MHC proteins display antigens more effectively than do others, in part contributing to the varied resistance of individuals to different pathogens.