How Allergies Occur
Hypersensitivity Reactions Are Due to a Number of Mechanisms
Despite being unpleasant and occasionally serious, allergy is really a normal reaction of the body's immune system.
Almost everyone has experienced some form of allergy, whether it's hay fever, an insect bite or a rash brought on by some foods or change of washing powder.
Although unpleasant and occasionally dangerous, allergy (sometimes called hypersensitivity) is a normal process of the body's immune system. Without the allergic response infection would wipe us out long before we reached adulthood. So even though allergy causes us problems now and then as individuals, it's worth it for the long term survival of our species.
Major Components of Allergy
- Lymphocytes: circulating white blood cells which produce antibodies. Some, (T lymphocytes) have a controlling role in the immune system.
- Antibodies: proteins produced by lymphocytes whose function is to kill invading micro-organisms, neutralise toxins and destroy foreign proteins. Often work in partnership with two types of cell -mast cells and macrophages.
- Antigen: anything that causes the production of antibodies which will then attempt to destroy the antigen. Could be a micro-organism, foreign protein, a drug or even the body's own tissue.
- Mast cells: large cells that hang around blood vessels. They contain tiny packets of very powerful chemicals which, when released, initiate inflammation and destroy micro-organisms. Unfortunately they also do a bit of damage to surrounding tissue.
- Macrophages: these cells have the job of engulfing and destroying insoluble matter in the blood such as bacteria or clumps of protein.
Types of Allergy
Allergies are divided into four types depending on which parts of the immune system are involved.
- Type I: examples of this are hay fever, nettle rash and asthma. The antigen can be pollen, dust mites or bee stings among many. The antigen causes antibodies to attach to mast cells which in turn release their powerful chemicals. Rarely, type I allergy may cause anaphylaxis, a whole body reaction involving circulatory collapse and sometimes death.
- Type II: occurs when a chemical, often a drug, binds to some of the body's cells. This makes those cells antigenic which of course then provokes attack by antibodies. Cells affected this way are often red blood cells which leads to a type of anaemia and platelets which causes bleeding – often under the skin giving rise to purple patches (purpura). Type II reactions are also involved in autoimmune disease.
- Type III: antibodies and antigens sometimes combine to form clumps which circulate in the blood. Macrophages have the job of mopping these up and destroying them. Occasionally there are too many clumps to deal with and these are inclined to lodge within the walls of blood vessels. This then provokes the production of chemicals to dissolve the clumps. In the process the blood vessel wall gets damaged, which if widespread may give rise to such conditions as lupus and glomerulonephritis.
- Type IV: sometimes called delayed hypersensitivity because whereas the above are pretty much immediate, occurring within minutes or a few hours, type IV usually manifests after one, two or more days. The mechanism for this reaction seems to involve T lymphocytes attracting macrophages to the area, the macrophages then secrete chemicals which set off an inflammatory reaction. Examples are contact dermatitis, some types of insect bite and the Mantoux test for TB.
This article is for information only. If you have any health concerns you should consult your doctor.
Resources:
Medicine. Ed. Souhami and Moxham. Pub. Churchill Livingstone 2002
An Introduction to General Pathology, Spector and Axford. Pub. Churchill Livingstone 1999
