WHAT ARE ALLERGIES?

An allergy is the result of an over-active immune system

Most people think of ‘food allergies’ when they think about allergy, but please go to the page listing common allergens (allergy-causing substances) for a broader view. We now know that we can react to pollens, chemicals in the air and in cosmetic body products such as make-up, shampoo or skin cream, preservatives and additives in our food and water, metals (even gold and silver) that we are in contact with through our jewellery and watches, and of course the food itself. Some people even react badly to natural gas used in cooking, traffic fumes, the light coming from fluorescent tubes and even to sunlight!

Our immune system is amazing! Its job is to identify foreign invaders [such as bacteria, viruses and toxic substances] and then destroy or excrete them. It can tell which cells have been infected with viruses, and it also identifies cells which over-grow, such as potential tumours.

It is constantly on guard and working hard. But sometimes it can be adversely affected by stress, medical drugs or too many toxic invaders and so it starts to make mistakes. The immune system can misidentify a harmless substance [like food] as dangerous, and then proceed to attack it with great vigour. Or it can also misidentify a harmful substance [such as mercury] and instead of ejecting it as soon as possible, it will store it somewhere in the body fat – often in the brain!

Almost any substance applied to the same area of skin often enough can induce allergy; so that eventually when it is applied to any other area in the same person, it will bring out a rash. In these cases, the non-protein substance attaches itself to one of the body proteins to form a complex which causes allergy. This is what can happen in the case of many drugs which have to be taken repeatedly, e.g. penicillin, sulphonamides, barbiturates (sleeping pills) and aspirin, all of which commonly cause side-effects.

WHAT HAPPENS INSIDE THE BODY?

In the past, people believed that the only true allergic reaction was to a protein in a substance foreign to the body, e.g. food. This protein is called the ‘allergen’ or ‘antigen’. The body’s immune system is constantly primed and ready to repel invaders in the form of bacteria and viruses, and one of the ways it recognises them is by the proteins on their surfaces. The body’s defences depend upon lymphocytes (white blood cells) whose job it is to recognise and defend against harmful antigens by producing antibodies that are specific to that particular antigen. The antibodies combine with the antigen and render it harmless, and then the immune system remembers the pattern of the protein so it can quickly produce antibodies again the next time.

In an allergy-prone person, the lymphocytes mistakenly react to a harmless antigen, say from wheat, as if it were a dangerous invader. They produce the necessary antibodies and nothing specific happens to the body the first time, but while the antibodies are waiting for the next ‘invasion’ they attach themselves to other tissues called mast cells. Mast cells are always on guard waiting to be needed in the case of tissue damage or infection, and they carry a plentiful supply of a number of biochemicals, including histamine.

Histamine has the effect of dilating (widening) blood vessels and increasing the permeability of the blood vessel walls in order to control the flow of fluid, and it also controls the production of fluid in the mucous glands. This happens on a subdued level all the time, without our knowing about it, but when the antibodies attached to the mast cells become excited by the arrival of a new invading protein, this causes the mast cells to dump their whole stock of histamine at once. The result of this is redness, heat, pain, swelling, itchiness and a plentiful flow of mucous – all signs of a typical inflammatory response to injury or infection – and also typical signs of an allergic reaction.

Allergies can be divided into types according to the speed and duration of the response :

1. Type 1 - where the symptoms are fairly instant and unmistakable every time the person comes into contact with the substance. Hay fever falls into this category, as does anaphylactic shock, asthma, hyperactivity and ADD, rashes, swellings etc. The reactions last 2-3 days and are mediated by Immunoglobulin E.

2. Type 2 - The first time you meet a substance it may be perfectly OK, but a reaction occurs on the second time of exposure. There is generally a localised response (e.g. hand or knee). It is mediated by Ig G or Ig M. You may be able to have a little of the allergen now and again, but if you do react to it, it may last 18-21 days and then improve.

3. Type 3 – is also activated on the second time of exposure, but the reaction is usually systemic (e.g. rash, burning or swelling all over the body). Mediated by Ig G or Ig M, it may last from 18-21 days. Narcolepsy can be caused by this.

4. Type 4 - Delayed allergy reaction, which normally goes unrecognised as the symptoms only appear hours, or even days, after contact with the substance. The reactions are usually not dramatic, and may even not be noticed. If the person is in contact with the allergen every day, the allergic reactions just form part of a generalised chronic condition, e.g. chronic inflammation, constipation, sinusitis, dandruff, dry skin, aches and pains, depression, hyperactivity etc. It is this delayed type of reaction that people often fail to associate with allergy. It does not show up on skin tests or blood Ig E antibody tests.

Whether instantaneous or delayed, these allergic reactions are caused by an inappropriate response from the body’s immune system when faced with a foreign substance, e.g. food, traffic fumes, lanolin. The reactions the body gives are similar to its reactions to tissue injury or infection, and (as described above) it releases histamine and other biochemicals into the cells that start a defensive response called inflammation.

If you cut yourself, it would be perfectly OK for the body to start an inflammatory process to clean and repair the damaged area (producing redness, pain, heat and swelling). However, it’s not OK for the body to have this reaction every time you eat a sandwich or breathe in some pollen!