specific body defences

• Lymphocytes are white blood cells involved in the specific immune response.
• 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.
• 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 or 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 which are harmless to the body. E.g. Pollen

  This hypersensitive response is called an allergic reaction.
• Apoptosis is programmed cell death.
• T lymphocytes destroy infected body cells by recognising antigens of the pathogen on cell membrane and inducing apoptosis.
• • T lymphocytes attach onto infected cells and release proteins. These proteins diffuse into infected cells causing the production of self-destructive enzymes which cause cell death.
  • The remains of the cell are then removed by phagocytosis.
• Failure of 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 leukaemia.
• • Some of the cloned B and T lymphocytes survive long-term as memory cells.
  • The development of memory cells provides immunity.
• • When a secondary exposure to the same antigen occurs, these memory cells rapidly give rise to a new clone of specific lymphocytes.
  • These destroy invading pathogens before an individual starts to show any symptoms.
• During the secondary response, antibody production is greater and more rapid than during the primary response.
• Human immunodeficiency virus attacks and destroys T lymphocytes. HIV causes depletion of T lymphocytes which leads to the development of acquired immune deficiency syndrome or AIDS.
• Individuals with AIDS have a weakened immune system and so are more vulnerable to opportunistic infections, such as pneumonia and influenza.
• Phagocytes engulf pathogens and display antigens and become antigen presenting cells. These activate T-lymphocytes.