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If pathogens pass the non-specific first line of defence, they will cause an infection. However, the body has a second line of defence to stop or minimise this infection. This is called the immune system, and mainly consists of two types of white blood cell called phagocytes and lymphocytes.

Phagocytes

Phagocytes are attracted to pathogens in the blood and bind to them. The phagocyte’s membrane surrounds the pathogen and engulfs it. Enzymes found inside the cell then break down the pathogen in order to destroy it. As phagocytes do this to all pathogens that they encounter, they are called non-specific.

Lymphocytes

Lymphocytes are another type of white blood cell. They recognise proteins on the surface of pathogens called antigens. Lymphocytes detect that these are foreign, ie not naturally occurring within the body, and produce antibodies. This can take a few days, during which time you may feel ill. The antibodies cause pathogens to stick together and make it easier for phagocytes to engulf them.

Some pathogens produce toxins which make you feel ill. Lymphocytes can also produce antitoxins to neutralise these toxins. Both the antibodies and antitoxins are highly specific to the antigen on the pathogen, therefore the lymphocytes that produce them are called specific.

If your body encounters a particular antigen, your lymphocytes might recognise it. If they do, they clone themselves in order to make enough antibodies to destroy the pathogen. Memory cells are also created which remain in your bloodstream and produce a quick response if the antigen is encountered again. You are then said to be immune.

The best immunity comes from having fought off a disease. However, in some cases such as a serious disease that could prove fatal on first infection, this is not an option. Instead vaccination is used.