Topic 5 - Cell recognition and the Immune system

• Antigens are molecules that can generate an immune response when detected by the body. They are usually found on the surface of cells and can be used by the immune system to detect pathogens, abnormal body cells (e.g. cancer), toxins and non-self material (e.g. from an organ transplant).
• The human body has a range of defences against pathogens. These include general, immediate defences such as the skin acting as a barrier, however some defences can be more specific and long-lasting. These involve lymphocyte cells.
• Cell mediated responses involve T lymphocytes. For example, phagocytosis is a cell mediated response.
• Humoral responses involve B lymphocytes. For example, clonal selection is a humoral response.
• Proteins have enormous variety and a highly specified tertiary structure. This 3D structure allows lymphocytes to distinguish between cells.
• Lymphocytes learn to recognise ‘self’ cells in the foetus. The lymphocytes constantly collide with other cells, and will collide almost exclusively with the body’s own material.
• Some of the fetal lymphocytes will have receptors complementary to the body cells, so these lymphocytes will either die or become suppressed. The only remaining lymphocytes are those which fit and respond to foreign material.
• Lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before maturing.