The original concept of allergy included all immune-mediated diseases and the
term allergy was interchangeable with the term "hypersensitivity." In one
textbook of clinical immunology allergy is defined: “The original definition of
allergy (Von Pirquet, 1906) was a specifically changed reactivity of the host to
an agent on the second or subsequent occasions. This covers a whole spectrum of
immune responses, both protective and harmful. However, more recently, the term
allergy had been restricted to the latter and is now synonymous with type 1
hypersensitivity. This rigid definition is too narrow to cover the range of
conditions seen by allergists in clinical practice. It is likely that all four
types of hypersensitivity are involved in various allergic diseases and
indistinguishable reactions can sometimes be produced without immunological
Four mechanisms of hypersensitivity are:
Type 1 or immediate hypersensitivity is IgE-mediated or common allergy.
Type 2 or cytotoxic reactions mediated by antibody, complement, and/or
cellular mechanisms. The target in type II reactions is a cell membrane and
cellular damage or death is the result.
Type 3 mechanisms involve antibodies forming immune complexes with antigen.
Circulating complexes activate complement, attach to red blood cells that are
removed by phagocytosis in the spleen, or leave the circulation and trigger
inflammation in tissue spaces (Arthus reaction), or are phagocytosed by
macrophages in tissue spaces which present antigen, release cytokines and
Type 4 are cell-mediated reactions take 12 or more hours to develop and are
based on antigen, T-cell interaction. Inflammation is the basic tissue
Allergy can be thought of as hypersensitivity disorders with external causes.
Substances that trigger allergic responses are antigens. These are often
proteins that can be found in air, food and water. Airborne antigens such as
plant pollens or house dust are well known. Other airborne antigens and food
antigens are less obvious. New and foreign substances introduced to the body
such as drugs and herbs usually cause delayed allergic reactions.
When investigating the origin of hypersensitivity diseases, food materials
should be given priority consideration since that are the biggest chunk of the
environment to get inside human bodies. If the term "food allergy" refers to all
interactions between molecules derived from the food supply and the immune
system, then many hypersensitivity disorders fall into the category of food
allergy. The spectrum of manifestations of food allergy can only be understood
if different patterns of food allergy are allowed.
The first distinction that recurs in the allergy literature is between
immediate and delayed patterns of allergic reactivity that correspond to
IgE-mediated allergy and non-IgE mediated responses.
Many authors refer to the original four categories of immune-mediated injury
defined by Gell and Coombs. The concept of four mechanisms is just a starting
point for understanding immune-mediated disease. These very complicated
defense-injury sequences cause a variety of disease states.
The immediate or type 1 allergy pattern is easily recognized because it
involves quick and dramatic symptoms. Hay fever is the most common type 1
allergy and can be diagnosed by allergy skin tests and by IgE antibody-antigen
tests such as RAST or ELIZA.
Delayed patterns of allergy are not so obvious and generally go unrecognized.
Allergy skin tests do not show this problem. Symptom onset is delayed many hours
after exposure to the trigger. Allergic reactions to drugs such as penicillin
and to foods involve delayed hypersensitivity.