The human immune system is an essential part of defense against infection; however the general defending immune system can occasionally produce harmful response to the host. Such responses are identified as hypersensitivity response, as well as the study of these is called immunopathology. This term came from the impression that the persons who have been formerly exposed to an antigen, marked noticeable responses to that antigen later and are consequently supposed to be sensitized. Some of the most common reactions to exogenous antigens cause the group of disease known as allergy which has variety of symptoms ranging from itching, rash, fever, asthma and anaphylaxis.