What is an allergy?
An allergy or hypersensitivity is one of the most common ailments that arises due to abnormal functioning of the immune system. It is a result of exposure to a specific antigen (allergen) by a sensitized person who has already developed antibodies against the allergen in question.
The interaction of the allergen with the antibody can result in an allergic reaction in many of these sensitized individuals. However, not ever person harboring specific antibodies to certain allergens develops an allergic reaction – some are only sensitized but it does not progress to an allergic reaction.
What are the types of allergic reactions?
There are four types of allergic (hypersensitivity) reactions based on the mechanism of the reaction. The types I, II, and III allergic reactions are antibody mediated reactions, while type IV is a T-cell mediated reaction.
Type I Allergic Reaction
The allergen is usually a soluble antigen which reacts with antigen-specific IgE antibodies already present in the body. This stimulates the mast cells (a type of immune cell) leading to its degranulation and the release of histamine. It results in a cascade of reactions with the sudden release of mediators of immune reactions (like cytokines and leukotrienes). The immediate effects in type I allergic reactions are explained by the activation of mast cells while the delayed effects are due to the slow stimulation of other immune cells like eosinophils and basophils.
Most of the common forms of allergic diseases, like anaphylaxis, asthma, allergic rhinitis, urticaria and so on, are type I allergic reactions.
Type II Allergic Reaction
Type II allergic reactions can result from the chemical alteration of antigens present on the surface of some of the cells (like red blood cells) by certain drugs. The alteration in the antigen on the cell surface can result in formation of antibodies (IgG) against these cells and ensuing destruction of the cells.
Penicillin or quinidine can result in hemolytic anemia due to alteration of the antigens present on surface of red blood cells. The subsequent production of antibodies against those antigens ultimately destroy the cells.
Type III Allergic Reaction
Type III allergic reactions result from the deposition of immune complexes containing soluble antigens along with antibodies (IgG) targeting these antigens in certain tissues. The deposition of these antigen-antibody immune complexes in the tissue stimulates a complement mediated immune reaction that result in damage of the tissue.
Serum sickness is an example of a type III hypersensitivity reaction. It arises following an injection of large quantities of soluble antigens (like horse serum). Serum sickness is usually characterized by chills, fever with rash, urticaria, joint pain and glomerulonephritis that develops few days after exposure to the antigen. Type III reactions are usually self-limiting.
Type IV allergic reaction
Type IV (delayed-type) allergic reactions are antigen-specific T-cell mediated reaction and classically known to appear after a time lag. In type IV reactions, the antigen is targeted by T-cells, unlike by the antibodies in other types of reactions. The reaction stimulates the release of mediators of inflammation which lead to tissue injury.
Contact dermatitis following exposure to certain metals and plants are examples of type IV hypersensitivity reactions.
What is Anaphylaxis?
Anaphylaxis is the medical term for a severe allergic reaction that can affect the entire body and is potentially life threatening. An anaphylactic reaction can lead to shock, hence the term anaphylactic shock. Anaphylaxis presents with the same signs and symptoms of shock in addition to an itchy skin rash, airway constriction, nausea, vomiting or diarrhea.
Anaphylaxis is known as a type I hypersensitivity reaction and occurs within 15 to 30 minutes of being exposed to the antigen (foreign organisms or substance). Sometimes a severe reaction can occur within seconds. This reaction is mediated by immunglobulin E (IgE), which is a type of antibody. Antibodies are developed by the immune system to help immune cells identify antigens as being dangerous and triggering a reaction against them. IgE tends to reside within the respiratory passages, skin and mucus membranes. Therefore much of the symptoms of this type of hypersensitivity reaction occurs in these areas.
Milder type I hypersensitivity reactions may be seen in acute allergies and chronic allergic conditions like asthma and hay fever. Anaphylaxis, however, is a much more severe reaction commonly triggered by certain drugs, foods or insect toxins (from bites or stings). Unlike a mild allergic reaction, anaphylaxis requires emergency medical attention so that epinephrine can be administered. Failure to do so can sometimes lead to death.