allergic reactions


hypersensitivity

An excessive reaction, local or systemic, or inappropriate immune response to an antigen. Four types of immune responses are usually described, but the main reaction involving the eyes is type 1. They are also called allergic reactions types 1-4.
type 1 hypersensitivity An immediate, abnormal reaction occurring when an antigen reacts with an antibody (e.g. immunoglobulin E (IgE)) attached to a mast cell or basophil. This leads to the release of specific chemical mediators of allergy (e.g. histamine) that react with target organs throughout the body. Systemic signs include: itching, lacrimation, skin rash and possibly haemodynamic collapse and shock. Allergic conjunctivitis is an example of this type of hypersensitivity.
Type 2 h . (cytotoxic h.) is caused by an interaction of antibody and antigens on cell surfaces. Examples: Graves' disease, myasthenia gravis. Type 3 h. (immune-complex mediated h.) is mediated by a combination of antigen-antibody. Example: systemic lupus erythematosus. Type 4 h. (T cell-mediated h.) is a delayed reaction (several days to develop) mediated by T lymphocytes. Example: rheumatoid arthritis. See antihistamine; mast cell stabilizers.