|The Allergy Center|
Immediate Allergy Type 1
Shrinking Definition of Allergy
The reason for the definition of allergy to shrink toward a narrowly-defined clinical practice probably was the skin test. If anything distinguished an allergist from his/her colleagues, it was the skin test. By a practical evolution of allergy practice, those clinical problems which were diagnosable by skin reactions became the special property of allergists. Allergy therapy became synonymous with desensitization (immunotherapy) injections.
But, the original definition of the term Atopy could apply to a larger, ever expanding list of immune-mediated strange diseases which plague us at the end of this century of extravagant biological misadventures. For some time, it has been appreciated that food allergy operates in a more complex and mysterious way than inhaled allergy. Although skin tests were used to test for food sensitivity, many allergists also prescribed various kinds of low allergy or elimination diets on clinical grounds with satisfactory results. Allergists generally appreciated that allergy shots containing food antigens were not helpful. Nevertheless, the IgE model was the easiest route to follow in the study of food allergy.
IgE Triggers Mast Cells and Basophils
Immediate or Type 1 patterns of allergy can largely be attributed to IgE and a sub-population of immune cells, the mast cells and basophils. These cells degranulate if sufficient antigen reacts with IgE antibodies which act as receptors on the cell's surface. An inherited tendency to make excessive amounts of IgE antibody is one characteristic of atopic individuals.
The most explosive, immediate, and obvious allergic reactions begin with allergens attaching to basophil and mast cells triggering their degranulation Allergens meet reactive cells just under the surface of the skin, respiratory, gastrointestinal and genitourinary tracts. Mast cells release three sets of mediators with dramatic, immediate, local and systemic effects.
When Pfizer pharmaceuticals launched their new antihistamine, Reactine (cetirizine), they spent a great deal of advertising money educating physicians about the late-phase response They claimed that Reactine was more successful in blocking the late-phase response than other antihistamines. The news to many physicians was that there was more to allergy than immediate reactions mediated by histamine. IgE-mediated reactions tended to recruit other immune responses which turned acute events into longer lasting and, possibly, more destructive immune-mediated disease.
The idea of a simple, linear type 1 reaction with histamine as the major mediator released has undergone significant evolution as tissue events have been studied in more detail. Even in the simplest tissue-model, the nasal mucosa reacting to a single exposure to pollen antigen, a biphasic response has been described. A late-phase reaction follows hours after the initial reaction. Mast cells release three sets of mediators with dramatic, immediate, local and systemic effects. The big players are histamine, prostaglandins, and cytokines, including leukotrienes B4, C4, D4, E4. All are bronchoconstrictors and increase permeability of blood vessels as in anaphylaxis and asthma. Three chemotactic factors recruit and activate other cells that extend the immune response from hours to days.
For reasons which are not well understood, a variety of patterns of cell recruitment occur in chronic inflammatory states - in chronic rhinitis and sinusitis, for example, there may be a predominately neutrophilic or eosinophilic response. Eosinophils are especially destructive to tissues; they release major basic protein that is damaging to cells. Neutrophils and basophils supply histamine-releasing factors, prostaglandins and leukotrienes which contribute further to local organ effects; bronchoconstriction and edema in the lung are typical effects in asthma triggered by inhaled antigens. These mediators tend to recruit lymphocytic responses which tend to sustain the type 4 or cell-mediated hypersensitivity state in target organs.
The complexity of food allergy begins with vagaries of antigen distribution, and extends to the lottery-like variables of cell-recruitment and target organ responses If the mast cell-IgE mechanism is thought of as a first-line triggering device whose main function is to recruit other immune responses, then the importance of these reactions is better understood.
Allergy shots are immunological treatments. The immune response to any reactive substance can be modified by giving repeated challenges of the reactive substances. Allergy shots for hay fever start with a serum containing the pollen antigens which caused positive skin responses. The antigens are administered in increasing concentrations by regular injections under the skin. It remains unclear how the shots work.
One response to the injected antigen is the production of a second antibody population of the IgG class. These IgG antibodies are thought to compete with IgE antibodies, blocking the allergic response. It is also possible that the antigen injections stimulate suppressor T-cells or inhibit helper T-cells and reduce production of IgE.
Allergy shots have limited therapeutic application. The hay fever sufferer and some asthmatics with specific inhalant reactions to grass pollens do well with desensitization. Immunotherapy also protects patients who have had anaphylactic reactions to bee and wasp stings. Patients with complex reactivity, food reactions and drug reactions do not do well with allergy shots, and the shots are not usually recommended. It is our policy to avoid allergy shots in patients who have food allergy and other forms of delayed immune responses.
Hear About Immediate and Delayed Patterns of Allergy