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Lymphocytes are the white blood cells involved in the specific immune response. They recognise specific antigens on invading pathogens. Antigens are molecules, often proteins, located on the surface of cells that trigger a specific immune response.

Lymphocytes detect that the proteins and pathogens are foreign - not naturally occurring within the body - and produce antibodies. This can take a few days, during which time you may feel ill.

The antibodies created by the lymphocytes cause pathogens to stick together and make it easier for phagocytes to engulf them.

Each lymphocyte produces its own specific receptor that corresponds to a specific antigen. When a lymphocyte encounters that antigen it multiplies, creating large numbers of identical lymphocytes bearing that particular receptor.

Some pathogens produce toxins which make you feel ill. Lymphocytes can also produce antitoxins to neutralise these toxins. This means that the toxins cannot bind to body cells and cause damage.

Antibodies and antitoxins are highly specific to the antigen or toxin that is made by the pathogen. That is why we say that the lymphocytes that produce them are specific.

There are two types of lymphocyte:

• B lymphocytes
• T lymphocytes