Doctors recognize four types of reaction in which the immune system responds to an antigen so strongly that unpleasant symptoms are caused. These are called hypersensitivity reactions. The reaction caused by mast cells and IgE is the most common and troublesome of the four, and is known as Type I hypersensitivity. The other three reactions involve different parts of the immune system. Type II hypersensitivity is not relevant here, while Type IV hypersensitivity is a very slow immune response produced by a particular group of immune cells. It may be involved in some reactions in the gut, such as Crohn’s disease but it is not generally relevant to food allergy. This section is therefore confined to Type III reactions. These occur when there is a substantial production of antibodies in response to an antigen in the blood. It is the sheer weight of numbers that causes the problem – die antigens and antibodies, bound together in immune complexes, are like so much litter going round in the bloodstream.
In the case of food, undigested molecules get into the blood through the gut wall, after a meal. This is a normal process that occurs in the healthiest of individuals, although in the food-sensitive person it is likely to be more pronounced because the gut wall is more leaky. Once the food molecules enter the bloodstream they encounter antibodies – again a natural, healthy process which leads to the formation of immune complexes when the antibodies and antigens bind together. Immune complexes attract the attention of phagocytes or ‘eating cells’, the vultures of the immune system that clear up any debris, dead cells and invading bacteria they come across.
Immune complexes form all the time, whenever antibodies encounter their antigen. Normally they are cleared from the blood by the phagocytes within a few hours. But if the immune complexes are both large and numerous, the phagocytes may not be equal to the task. Then the immune complexes accumulate in the blood, and eventually they are deposited in the blood vessels. This is the condition known as Type III hypersensitivity or serum sickness.
Serum sickness happens in autoimmune diseases, such as SLE (systemic lupus erythematosus, or lupus; where there are a great many antibodies to the patient’s own proteins circulating in the blood. It is also believed to happen, to a lesser degree, in rare cases of food allergy – in some types of kidney disease, for example, where the disease seems to be induced by food. Although there is no definite proof for this, the circumstantial evidence is quite strong.
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