The human body must constantly fight its enemies. It does this through the immune system which can target the body’s key enemies including defective body cells and foreign invaders. The latter include bacteria and viruses which can enter the body through a number of ways. The most common entry points are through the two systems which are designed to allow entry: alongside food into the digestive system and alongside air into the lungs.
The digestive system, in particular, has a specially developed immune system called the ‘gut-associated immune system”. This is designed to let food particles be digested and further processed. Therefore things like amino acids, fatty acids and simple sugars are considered safe by the immune system and can pass through the gut wall into the bloodstream and the rest of your body. Problems with this system can be the cause of immune problems like allergies. Similarly, the nasal passages can filter out unwanted particles. Mucous membranes in both the nasal and digestive systems are therefore considered the first line of defence against bacteria and viruses.
If that fails, the immune system itself is located throughout your body to fight invaders wherever they may have entered. The immune system contains organs devoted to supporting it, including the spleen, the thymus, bone marrow and the lymphatic system, as well as specialised cells, including white blood cells, and chemical compounds including antibodies.
These different parts of the immune system work together to keep you safe now and in the future. That includes keeping a kind of “record” of past infections through memory cells. Memory cells are special white blood cells known as B- and T-lymphocytes. When the immune system has this record it can recognize an infection and act more quickly. This is how immunity to certain diseases works, such as chickenpox.
However, unfortunately, this system does not work for all diseases. Some infections, including the common cold and the winter flu, are caused by a large number of similar but slightly different bacteria or viruses. For example, the flu virus is slightly different each year, meaning that even if you have fought it off in one year, you can get it again in the future.
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