The immune system is the body's defence against bacteria, viruses and other foreign organisms or harmful chemicals. It is very complex and has to work properly to protect us from harmful bacteria and other organisms in the environment, which may infect our bodies.
The immune system has three parts:
- the skin and mucosal membranes
- soluble proteins produced by the body, which are present in body fluids
- specialised blood and lymph cells.
The skin and mucosal membranes
The skin and the lining of the body cavities that open to the outside, such as the gut and the reproductive tract, are protective barriers that prevent invasion by any harmful organisms.
The oil or sebum produced by the skin keeps it slightly acidic, which controls the growth of any organisms on the skin.
Cells in the lining of body cavities produce fluids such as saliva in the digestive system and mucus in the lungs, which provide a defence against harmful organisms.
The body also has its own natural and helpful organisms that prevent other more dangerous organisms from taking over.
Special proteins and chemicals are present in body secretions and fluids. If organisms do manage to enter the body, the proteins attack them in various ways, often by dissolving their protective outer layer.
Specialised blood and lymph cells
Specialised cells are produced by the bone marrow (the spongy cells in the middle of some of our bones) and the lymphatic system. The lymphatic system is a complex system made up of organs such as bone marrow, the thymus, the spleen and lymph nodes. Lymph nodes are found throughout the body and are connected by a network of tiny lymphatic vessels.
Different types of white blood cells play varying roles in killing organisms that invade the body. They arrive at the site of damage or infection and destroy these organisms (known generally as antigens).