specific cellular defences against pathogens

• Lymphocytes are the white blood cells involved in the specific immune response - there are two types, B lymphocytes and T lymphocytes.
• All lymphocytes respond to specific antigens on invading pathogens - antigens are proteins on the surface of cells that trigger a specific immune response.

  All lymphocytes have a single type of membrane receptor which is specific for each antigen. The binding of the antigen to the membrane receptor leads to repeated cell division of a specific lymphocyte for that antigen which results in a clonal population of identical lymphocytes which will all carry out a specific response against that particular antigen
• B lymphocytes produce antibodies against antigens.

  Antibodies are Y-shaped proteins that have receptor binding sites specific to a particular antigen on a pathogen.

  Antibodies bind to the antigens which inactivates the pathogen. The resulting antigen-antibody complex can then be destroyed by phagocytosis.
• B lymphocytes can respond to antigens on substances that are harmless e.g. pollen. This hypersensitive response is called an allergic reaction

  Examples of allergic reactions - hay fever and allergic asthma. In severe allergic reactions, anaphylaxis can occur.
• T lymphocytes destroy infected body cells by recognising antigens of the pathogen on the cell membrane of the infected body cell membrane. This induces apoptosis.

  Apoptosis is programmed cell death. T lymphocytes attach onto infected cells and release proteins which diffuse into the infected cells. This produces self-destructive enzymes which cause cell death.

  The remains of the cell are 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 which causes autoimmune diseases.

  In autoimmunity, the T lymphocytes attack the body's own cells and causes diseases such as type 1 diseases and rheumatoid arthritis.
• Some of the cloned B and T lymphocytes from the specific immune response survive long-term as memory cells.

  When exposure to the same antigen occurs, this is called the secondary response. During this, antibody production is greater and more rapid than during the primary response because of the memory cells produced in the primary response. The memory cells give rise to a new clone of specific lymphocytes, all of which produce the appropriate antibody. These clones destroy the invading pathogens before the individual shows any symptoms.
• The human immunodeficiency virus (HIV) attacks and destroys T lymphocytes.

  HIV causes the depletion of T lymphocytes which leads to AIDS. People with AIDS have a weakened immune response and so are more vulnerable to infections.