Specific defence

Allergy or example eg hayfever B lymphocyte. (1) Attack/respond to a harmless antigen. (1)

Autoimmune disease or example eg arthritis T lymphocyte. (1) Attack/respond to self-antigens. (1)

Lymphocytes are the white blood cells involved in the specific immune response. Lymphocytes respond to specific antigens on invading pathogens.

Lymphocytes have a single type of membrane receptor which is specific for one antigen. Antigen binding leads to repeated lymphocyte division resulting in the formation of a clonal population of identical lymphocytes.

Antigens are molecules, often proteins located on the surface of cells that trigger a specific immune response.

B lymphocytes produce antibodies against antigens and this leads to the destruction of the pathogen.

Antibodies are Y-shaped proteins that have receptor binding sites specific to a particular antigen on a pathogen. Antibodies become bound to antigens, inactivating the pathogen. The resulting antigen-antibody complex can then be destroyed by phagocytosis.

B lymphocytes can respond to antigens on substances that are harmless to the body, eg pollen. This hypersensitive response is called an allergic reaction.

Failure of the regulation of the immune system leads to T lymphocytes responding to self-antigens. This causes autoimmune diseases. In autoimmunity, the T lymphocytes attack the body’s own cells. This causes autoimmune diseases such as type 1 diabetes and rheumatoid arthritis.

T lymphocytes attach onto infected cells and release proteins. These proteins diffuse into the infected cells causing production of self- destructive enzymes which cause cell death. The remains of the cell are then removed by phagocytosis. T lymphocytes can normally distinguish between self-antigens on the body’s own cells and non-self-antigens on infected cells.

T lymphocytes destroy infected body cells by recognising antigens of the pathogen on the cell membrane and inducing apoptosis. Apoptosis is programmed cell death.

Some of the cloned B and T lymphocytes survive long-term as memory cells. When a secondary exposure to the same antigen occurs, these memory cells rapidly give rise to a new clone of specific lymphocytes. These destroy the invading pathogens before the individual shows symptoms.

During the secondary response, antibody production is greater and more rapid than during the primary response.

The human immunodeficiency virus (HIV) attacks and destroys T lymphocytes. HIV causes depletion of T lymphocytes which leads to the development of AIDS (acquired immune deficiency syndrome).

Individuals with AIDS have a weakened immune system and so are more vulnerable to opportunistic infections like colds.