Allergy is defined as altered reaction to the antigen antibody interaction in a person Who is hypersensitive to an offending substance. Allergy is therefore also called hypersensitivity. A person may be hypersensitive to a variety of substances such as chemicals, dust, pollen-grain, hay, fungal spore, fungal hyphae etc. A person who is sensitive to any of these come in contact will produce immediate immunological reaction. The reactions that are adverse are either mediated by (a) humoral antibody or (b) cells.
The humoral antibody reaction produces immediate hypersensitivity while the cells produce delayed hypersensitivity. The antigens which produce hypersensitivity is referred to as allergens or sensitizing antigens.
The cell-mediated hypersensitivity or allergy is characterised by the following :
1. It is a delayed type of reaction begins only hours or days following the introduction of shocking dose of allergens.
2. The reaction subsides after 24-72 hours.
3. There is no specific site of reaction. Any vascular tissue may be affected.
4. Instead of such reaction, T-lymphocytes may he involved. The T-Lymphocytes probably carry fix immunoglobin recognition sites on their surface which react in the presence of allergens by releasing several soluble substances which trigger various changes in cells of other types to produce hypersensitivity.