Cells of the Immune System

B. Cells of Immune System : Two main categories of cell carry out the work of the acquired immune system: lymphocytes and antigen-presenting cells.

a. Lymphocytes : Lymphocytes, like other blood cells, arise from stem cells (haemocytoblasts) present in the foetal liver and, in adults, in the bone marrow. Some of these differentiate in gut-associated bursal lymphoid tissue (tonsils, Peyer's patches) and are called B-lymphocytes (B-cells); others differentiate in the thymus gland and are called T-lymphocytes (T-cells). Mature lymphocytes circulate in blood and lymph, though many settle in the lymph nodes. B-cells form the humoral or antibody-mediated immune system (AMIS), while T-cells form the cell-mediated immune system (CMIS) — both need antigens to be activated, but they respond in different ways.

Mechanism of response of T-lymphocytes to antigens : When a T-lymphocyte meets an antigen, it clones itself into four functionally distinct types: helper T-cells, which secrete lymphokines that drive proliferation of other T-cells and stimulate B-lymphocytes and macrophages; killer (cytotoxic) T-cells, which bind directly to infected cells, tumour cells or invading microbes and secrete perforins that punch a hole in the target cell's membrane, killing it (hence 'cytotoxic'); suppressor T-cells, which damp down the immune response to prevent it attacking the body's own cells; and memory T-cells, sensitised cells that persist long-term, ready to respond faster if the same antigen returns.

Mechanism of action of B-lymphocytes to antigens : A B-lymphocyte can be sensitised directly by an antigen or with the help of a helper T-cell. Once activated, it multiplies rapidly into a clone of plasma cells and memory B-cells. Plasma cells manufacture specialised glycoproteins called antibodies, which circulate through the blood and lymph, either bound to a cell membrane or free. Free antibodies perform three main jobs: agglutination — clumping bacteria/viruses into a mass that phagocytes can then engulf; opsonisation — coating bacteria to make them easier for macrophages to phagocytose; and neutralisation of toxins released by bacteria (e.g. tetanus toxin). Each antibody is specific to one particular antigen.