Specific Cellular Defenses

• Any foreign molecule that is located on the surface of cells (often a

  protein) which triggers a specific immune response is called an antigen, examples include;

  • Viruses

  • Bacteria

  • Bacterial toxins

  • Molecules on the surface of transplanted cells

  • Molecules on the surface of cancer cells
• Lymphocytes

  The body has an enormous number of different lymphocytes. Each has several copies of a single type of antigen receptor on its cell membrane.

  This antigen receptor is specific for one antigen.

  Antigen binding leads to repeated lymphocyte division resulting in the formation of a clonal population of identical lymphocytes.
• When an antigen receptor is activated by the binding of an antigen, the lymphocyte is said to be selected by the antigen. The lymphocyte then repeatedly divides, resulting in 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 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, e.g. pollen. This hypersensitive reaction response is called an allergic reaction.
• T lymphocytes destroy infected body cells by recognising antigens of the pathogen on the cell membrane and inducing apoptosis. Apoptosis is programmed cell death.
• T lymphocytes attach to 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. Failure of the regulation of the immune system leads to T lymphocytes

  responding to self-antigens. This causes autoimmune disorders.
• In autoimmunity , the T lymphocytes attack the body’s own cells. This causes autoimmune diseases such as type 1 diabetes and rheumatoid

  arthritis.
• 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 pathogen before the individual shows symptoms. During a secondary response, antibody production is greater and more rapid than during the primary response.
• AIDS is an acquired immune deficiency disease caused by HIV

  (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.