Immunology is the scientific study of the body's resistance to invasion by other organisms (i.e., immunity). 

In a medical sense, immunology deals with the body's system of defense against disease-causing micro-organisms and with disorders in that system's functioning.

Proliferating micro-organisms in the body cause many infectious diseases.

The body has certain chemical and cellular components that recognize and destroy foreign substances (antigens) within the body.

This new understanding led to highly successful techniques of immunization (q.v.) that could mobilize and stimulate the body's natural defenses against infectious disease.

Formation, mobilization, action, and interaction of antibodies and antigen-reactive lymphocytes, which are the two main active elements of the immune system.

Immunology also encompasses the increasingly important study of autoimmune diseases, in which the body's immune system attacks some constituent of its own tissues as if it were a foreign body.
The study of immune deficiencies has become an area of intensive research since the appearance of AIDS (acquired immune deficiency syndrome), a disease that destroys the body's immune system and for which there is currently no cure.

Medical and Alternative Medical Terminology, Explanations -  by Courtesy of Encyclopedia Britannica® 2001