Hypersensitivity (Allergy)
Hypersensitivity or allergy is the excessive immune response to common antigens, called allergens.
Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.
Common examples of allergens are mites in dust, pollens, animal dander, etc.
Symptoms of allergic reactions include sneezing, watery eyes, running nose and difficulty in breathing.
The use of drugs such as antihistamines, adrenalin and steroids quickly reduce the symptoms of allergy.
Commoeofd asthma.
Hay fever is the form of allergy due to pollen of grasses, trees and other plants. It is characterized by inflammation of the membrane lining the nose and sometimes of the conjunctiva.
In asthma, the tissues surrounding the bronchioles of the lungs swell up, and compress the bronchioles. Hence, there is difficulty in breathing.
Types of hypersensitivity reaction
They are divided into four classes (Type I-IV) based on the mechanisms involved and the time course of hypersensitive reaction. Type I hypersensitivity is an immediate or anaphylactic reaction, often associated with allergy Symptoms can range from mild discomfort to death. It is mediated by IgE, mast cells and basophils.
Type lI hypersensitivity occurs when antibodies bind to antigens on the patient's own cells marking them
for destruction. This is also called antibody-dependent (or cytotoxic) hypersensitivity, and is mediated by
IgG and IgM antibodies
Type IIl hypersensitivity involves circulating antigen-
antibody immune complexes that collect in various tissues and post capillary venules, with subsequent
complement fixation.
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Type IV hypersensitivity (also known as cell mediated or delayed type hypersensitivity) usually takes two to three days to develop. Type IV reactions
are involved in many autoimmune and infectious diseases, but may also involve contact dermatitis (poison ivy). These reactions are mediated by T cells, monocytes and macrophages. Anaphylaxis (anaphylactic shock) : It is an
allergic reaction involving all the tissues of the bodyand andurs in a few minutes after the injection of an antigen such as penicillin. Such a reaction is unevery serious ine Histamine released from ruptured mast cells causes marked dilation of all the arteries so that a large
amount of fluidis passed from the blood to the tissues like and there is a drastic fall in blood pressure
ust.
