All living organisms are continuously exposed to pathogenic
agents from the surrounding milieu, e.g. viruses, bacteria,
fungi and parasites. Skin and mucous membranes form efficient
barriers to these agents, but sometimes this defense is overcome.
If an infectious agent succeeds in penetrating into the
"milieu int€rieur", at first granulocytes and mononuclear
phagocytes become involved in the elimination of the intruding
microorganisms. This part of the defense system is largely
aspecific. Apart from this, vertebrates have a well developed
immune system which can mount specific immune responses to invading
organisms and foreign substances. The specificity of
the immune reaction is based on the presence of receptors on
the individual lymphocytes which recognize specifically any
one of many foreign substances. Only after the recognition of
the immunogenic material is a lymphocyte activated to perform
its function. Immunogenic substances or antigens are operationally
defined by their capacity to induce an immune response.
The recognition of a particular antigen by the receptors of an
individual lymphocyte is predetermined, i.e. the diversity of
lymphocytes, each specific for one of the numerous imaginable
foreign substances is generated before encounter with immunogenic
material. In addition to the specificity of lymphocytes
for a particular antigen, lymphocytes must also discriminate
between self and non-self. If they should fail to do so, an
immune response to tissues of the individual's own body would
arise, leading to autoimmune disease (Burnet, 1972)