Topic 2: - C: Cells and the immune system

• Antigens: molecules (usually proteins) that can generate an immune response when detected by the body.
• Antigens are usually found on the surface of cells, including all your body cells
• Antigens that aren't normally found in the body are referred to as foreign antigens - it is these antigens that the immune system usually responds to
• Antigens allow the immune system to identify:

  1. pathogens
  2. abnormal body cells
  3. toxins
  4. cells from other individuals of the same species
• Pathogens: these are organisms cause disease e.g. bacteria, viruses and fungi. All pathogens have antigens on their surface - these are identified as foreign by immune system cells, which then respond to destroy the pathogen
• Abnormal body cells: cancerous or pathogen-infected cells have abnormal antigens on their surface, which trigger an immune response
• Toxins: poisons, also molecules not cells. Some toxins are produced by bacteria - the immune system can respond to toxins, as well as pathogens that release them
• cells from other individuals of the same species: hen you receive cells from another species, such as in an organ transplant or blood transfusion, those cells will have some antigens that are different to you own (unless the donor is genetically identical to you). The foreign antigens trigger an immune response. This response leads to rejection of transplanted organs if drugs aren't taken to suppress the recipient's immune system
• For blood transfusions, the most important antigens are the ABO blood group antigens - if the donated blood contains A or B antigens that aren't recognised by the recipient's immune system, they will generate an immune response