Immunology is the study of the body's defence system. The system has evolved to prevent harmful organisms from gaining access to the body or to destroy them once they have entered, in order to prevent serious diseases.

• Non-specific mechanisms; such as physical barriers, movement, temperature, chemicals and phagocytic cells.
• Specific mechanisms under the control of a group of cells called lymphocytes. These cells and their products are highly specific in that they can recognise foreign cells and mount a response against them. This is referred to as an immune response.
  Lymphocytes may be subgrouped into:

1. T lymphocytes (T cells) that may develop into T helper cells, T killer cells, T memory cells and T suppressor cells.
2. B Lymphocytes (B cells) that may develop into plasma cells and B memory cells. Plasma cells develop from B cells that have become activated following the recognition of a foreign cell or antigen. They produce specific proteins called antibodies or immunoglobulins which recognise and bind to the foreign antigen that had elicited their production.

• Immunodeficiencies or an underactive immune system.
  The tests include assessment of humoral immunity by measurement of immunoglobulin levels, total complement activation and individual complement components (C3 and C4); cellular immunity via assessment of lymphocyte surface markers. T cell subsets are measured as part of monitoring HIV patients .
• Autoimmune diseases or an overactive immune system.
  Tests include autoantibody measurement for rheumatoid arthritis, connective tissue diseases such as systemic lupus erythematosus, Sjogren's syndrome, vasculitides such as Wegener's granulomatosus, the antiphospholipid syndrome and organ specific autoimmune diseases such as autoimmune thyroid disease; Goodpasture's syndrome, coeliac disease.
• Allergic disease.
  Tests for allergy include assays for allergen specific IgE, total IgE and eosinophil cationic protein.
• Cancer, especially involving the immune system.
  This includes serum and urine tests for screening and monitoring of multiple myeloma, macroglobulinaemia and light chain disease; lymphocyte surface markers and DNA studies for diagnosis, classification and subtyping of lymphoma and leukaemia.