4.5.2 - The immune system

Primary defences of the body: 

• Skin- Acts as a physical barrier, blocking pathogens from entering the body. Also acts as a chemical barrier by producing chemicals that are antimicrobial and can lower pH, inhibiting the growth of pathogens 
• Mucous membranes- Protect body openings that are exposed to the environment. Some membranes secrete mucus- a sticky substance that traps pathogens and contains antimicrobial enzymes 

If a pathogen gets past the primary defences, the immune system will respond. An immune response is the body's reaction to a foreign antigen. Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells. When a pathogen invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system. There are 4 main stages in the immune response:

• Phagocytes engulf pathogens- A phagocyte is a type of white blood cell that carries out phagocytosis (engulfing of pathogens). They are found in the blood and tissues and are the first cells to respond to a pathogen inside the body. A phagocyte recognises the antigens on a pathogen. The cytoplasm of the phagocyte moves round the pathogen, engulfing it. The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte. A lysosome (an organelle that contains digestive enzymes) fuses with the phagocytic vacuole.…