The Immune System

Primary Defences

Skin

Acts as a physical barrier, blocking pathogens from entering the body. It also acts as a chemical barrier by producing antimicrobial chemicals and can lower pH, inhibiting the growth of pathogens. 

Mucous Membranes

These protect body openings that are exposed to the environment. Some membranes secrete mucus which traps pathogens and contains antimicrobial enzymes. 

The Immune Response

An immune response is the body's reaction to a foreign antigen. Antigens are molecules found on the surface of the cells. When a pathogen invades, antigens on its cell surface membrane are identified as foreign which activates the immune system. 

Main Stages

• 1. Phagocytosis

A phagocyte is a type of white blood cell that carries out phagocytosis. They are found in the blood and in tissues and are the first cells to respond to a pathogen inside the body. 

• A phagocyte recognises the antigens on a pathogen.
• The cytoplasm moves around the pathogen, engulfing it.
• The pathogen is now contained in a phagocytic vacuole in the cytoplasm. 
• A lysosome fuses with the phagocytic vacuole. The enzymes break down the pathogen and it is absorbed into the cytoplasm. 
• The phagocyte…