User:Kinkreet/Immunology/Hypersensitivity

Hypersensitivity is where the body respond to non-infectious antigens in a harmful way, causing tissue damage and diseases. In 1963, P. H. G. Gell and Robin Coombs classified hypersensitivity into four types: Allergy (type I), cytotoxic and antigen-dependent drug allergy and urticaria (type II), immune complex disease (e.g. serum sickness and Arthus reaction) (type 3) and antibody-independent, delayed-type hypersensitivity (e.g. contact dermatitis) (type IV). The classification of diseases depends on the mediators of each response, the antigen which triggers the response, and the effector mechanism which causes harm.

Genetic susceptibility
Although environmental factors may provide many of the factors in hypersensitivity, variations in more than 100 genes alters the susceptibility of an individual to hypersensitivity. Many of these genes encode for components in the immune system. The gene ADAM33 is used in remodelling and bronchial smooth muscle contraction, and so is important in late-stage asthma. Mutations which leads to the down-regulation or function deletion of negative control molecules, or the amplification of activation, may lead to hypersensitivity. Mutations in genes such as IL-4RA, FcεRI-b, MHC, cytokines, CD14, T-bet, Gata-3 and Foxp3 can lead to hypersensitivity.

Environmental factors
Allergies are a combination of environmental and genetic factors. Environmental factors include route, dose and age of allergen exposure, urban environment, exposure to pollutants, maternal IgE, infections, siblings, day care centres, farm environments etc. It may be true that if one is exposed to pro-inflammatory environment (TH1 response), then it is less likely to develop hypersensitivity; this is also known as the hygiene hypothesis.

Common Allergens
Allergens such as drugs (mostly β-lactam antibiotics), serum, venoms and peanuts, can cause the release of mediators from white blood cells that cause inflammation and swelling. IgE binds to antigens and activates FcεRI receptors on mast cells and basophils; this causes their degranulation, releasing the vesicular inflammatory factors, such as histamines.

Allergens such as animal hair, animal fluids and insect bites are common causes of urticaria. They causes the release of histamine by mast cells and leads to the appearance of pale red, raised, itchy rash commonly referred to as hives. Fel d 1 is a protein that in cats is encoded by the CH1 (chain 1/Fel d 1-A) and CH2 (chain 2/Fel d 1-B) genes, and is present primarily in cat saliva; Fel d 1 allergy is the most common allergen in animal allergies. Fel d 1 causes IgG or IgE reactions. Similarly, in house dust mites, Der p 1 and Der p 2, cysteine proteases, causes similar reactions.

Pollen, dust mites can cause allergic rhinitis (hayfever); dander, pollen and dust mites can trigger asthma; there are also be food allergies caused by nuts, milk, eggs and fish.