This is the technical bit – I apologise because it is complicated but it is the very essence of the allergic reaction than enables us to understand why we react the way we do.
When a foreign substance, called an antigen, enters the body, the immune system manufactures specific antibodies that attach to and neutralize the antigen; however, for those with allergies, a normally harmless substance such as grass pollen, peanuts, or wasp venom can cause an immune response that results in their allergy symptoms.
Antigens that cause allergic reactions are known as allergens. The first time an allergy-prone person is exposed to an allergen, he or she makes large amounts of the corresponding IgE, rather than the IgG, IgM or IgA antibody. IgE is just a molecule that forms a specific attachment to the particular allergen. Like other antibodies, each IgE antibody is allergen specific; one reacts against grass pollens, another against ragweed.
These IgE molecules attach to the surfaces of mast cells (in tissue) and basophils (in the blood). Mast cells are plentiful in the lungs, skin, tongue, and linings of the nose and intestinal tract. When IgE antibodies sitting on a mast cell or basophil are “cross-linked” by their specific allergen, the mast cell or basophil release the powerful chemicals stored within their granules. These chemical mediators, including histamine, cause the allergy symptoms such as wheezing, sneezing, runny eyes and itching. The great majority of allergic reactions are of course limited in severity and usually just a minor inconvenience.
This, then, is the essence of an allergic reaction – cells in the body releasing histamine and other chemicals which then cause the symptoms we know as allergic reactions. A local allergic reaction just affects the cells in the immediate vicinity of the allergen – a system wide allergic reaction is caused by a cascade of cells disintegrating throughout the body. It is in this event that things can become really dangerous as the released chemicals start to cause, amongst other things, the veins and arteries to dilate and airways narrow: the potential for death then becomes quite apparent as blood pressure falls and breathing becomes laboured.
It is important to understand that if you have never encountered an allergen before, then you cannot be allergic to that allergen. You have to be sensitized first of all before you can be allergic.
The venom in stinging insects from the first sting would stimulate the development of a venom specific IgE antibody and it is that IgE mediator in the blood which is there awaiting the next sting and so “sparks off” the allergic reaction – either limited locally to the sting site or less frequently systemically around the whole body. It is the combination of the latent IgE in the blood and body tissues combined with the introduction of the next dose of allergen that causes the mast cells in the body to release histamine.
The RAST blood test can in principle tell if you are allergic by the presence of venom specific IgE in the blood.
Biting insects seek to insert their sucking instrument into skin and to facilitate their sucking process the salivate into the sucking instrument which limits blood coagulation. An allergic reaction to biting insects such as horsefly and mosquito is therefore a result of reaction to the saliva. It is rarely more than just an local allergic irritation due to the very tiny dose of saliva that is introduced.