(immunology) The system by which the body overcomes infections and other invasions by foreign bodies. Natural, or non-specific, immunity works by phagocytosis and by the action of the protein interferon. Specific rmmumtv, ie. a response to one particular infective agent (all of which, whether viruses, bacteria, parasites, or non-living particles arc known as antigens in this context), is most highly developed in vertebrates, including humans. It is of two kinds: humoral immumtv, in which antibody molecules circulate in the lymph and blood; and cell-mediated immunity, in which lvmph cells directly bind to the antigens. In humoral immunity, the arrival of the antigen stimulates the appropriate line of B lymphocytes in the bone marrow; these cells, and their descendants in the lymph nodes and spleen, produce the specific antibody molecules which bind on to the antigens, either making them incapable of infective action or holding them in lumps so that they can be easily engulfed and dissolved by the large macrophagc cells (this is facilitated by a group of proteins collectively known as complement, activated by the antibody-antigen complex). Cell-mediated immunity is similar, with the antigen binding being done by T lymphocytes which originate in the thymus gland. The humoral immunity produced by circulating antibody is most effective against infections by bacteria and viruses while they arc outside the cells; cell-mediated immunity is better against viruses inside cells, parasites, cancer cells and foreign tissue. See immunisation.
