An antigen is a substance that provokes immune response and reactivity in the body and in health, the immune system effectively protects against the foreign and infectious antigens that it encounters. Sometimes, however, it reacts inappropriately to them with either a deficient or an exaggerated ‘hypersensitive’ response. There are four different types of hypersensitivity, three of which are mediated by antibodies and one by T lymphocytes. An allergen