immune system

In humans, the immune system consists of many different parts with many different types of cells. Broad categories of cells are the phagocytes and lymphocytes, whilst broad categories of chemicals are the cytokines, interleukins and the complement system.

The function of all these cells is to protect the body against foreign invaders such as bacteria, fungi, viruses and multicellular parasites. In addition it also protects the body against itself by destroying tumours (auto-toxicity).

Occasionally, the immune system overreacts and attacks normal cells. This results in an auto-immune disease.

Also see: white blood cell, antigen, antibody, vaccine, acquired immunity, autoimmune reaction, immunoglobulin, AIDS

For more than you probably need to know go read http://gsbs.utmb.edu/microbook/ch001a.htm

The part of the immune system that causes allergies has to do with histamine. Anyone who has allergies is familiar with the effects of this substance.

As a system, the histamine system pre-dates the other aspects of the immune system--it is very primitive. Yet it did not evolve without a purpose.

It is believed that the histamine response arose in order to fight large parasites, common many, many, millenia ago, that were a threat to the existence of the organisims that eventually became us.