The main role of the immune system is to prevent disease caused by infection.
Infections can be caused by a wide variety of pathogens,
including bacteria, fungi, parasites (such as malaria) and viruses (such as influenza and the common cold).
The immune system comprises of a network of cells and tissues that work together to prevent disease. All parts of the body that interact with the external environment (such as the lungs, digestive tract and skin) have to be prepared to protect the body. The skin acts as a barrier to prevent pathogens entering the body. If you cut your leg, your skin protective barrier is broken and so bacteria are now able to enter your cut. Luckily, our bodies have an army of immune cells waiting for external invaders. These immune cells are called macrophages. They are able to recognise the bacteria as a threat, based on molecules the bacteria express on their surfaces. Once the macrophages recognised the bacteria, they can destroy the bacteria by eating (engulfing) it. This is a process called phagocytosis. At the same time, the phagocytes produce molecules called cytokines and chemokines. These alert the rest of the immune system to the presence of an invader. Different immune cells called neutrophils are then recruited to your leg. Along with the macrophages, they help to destroy the bacteria. Both the macrophages and neutrophils are part of the innate immune system. They provide the first line of defence against invading pathogens. If the invaders cannot be controlled by the innate immune system, then more specialised immune cells called lymphocytes are recruited. Lymphocytes include T-cells and B-cells. They work together to produce molecules like antibodies, which help to clear the infection. T and B cells are part of the adaptive immune system. Once these cells are activated, they are able to remember an infection. So that the next time the bacteria enters the body, they are able to be destroyed more rapidly. The innate and adaptive immune systems work together to fight off infections, and prevent the development of disease.
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