Complex network of specialized cells and organs that has evolved to defend the body against attacks by foreign invaders.

The reaction by the immune system to antigens (substances such as bacteria and viruses) that the body recognizes as foreign is called immune response.

The immune response can be divided into several phases:

1. the "innate" first response, mediated by cells able to destroy and engulf a large range of foreign organisms;
2. the secondary, "adaptive" response, characterised by the generation of antibodies and T cells that are specific for the antigen; and
3. a third, "suppression" phase, where the production of immune cells reverts to normal, and the information necessary to mount a future immune response to that antigen is retained in bone marrow memory cells.