Allergic Reaction: What It Is and What Types There Are

An allergic reaction is the immune system's response to a substance that is harmless to most people. It is triggered by repeated contact with an irritant already familiar to the body and can affect the skin, eyes, airways, and gastrointestinal tract. Below: the mechanism, the forms, and the signs that require a doctor.

Allergy: what it is and who an allergic person is

Allergy is a hypersensitivity that the adaptive immune system develops in response to non-infectious environmental substances. These are called allergens, and the reaction to them is called abnormal: for other people, the same pollen or milk is harmless. An allergic person is someone in whom sensitivity has already formed, often to several irritants at once.

The term was introduced by the Austrian pediatrician Clemens von Pirquet in 1906 while studying serum sickness and the reaction to tuberculin. The idea that the immune system, protective by nature, can harm the body was initially met with skepticism.

At least one allergic disease affects 8–10% of people worldwide, and in some regions of Russia at the beginning of the century, 10–35% of the population.

How the immune system causes an allergic reaction

Normally, immunity fights microbes, but in allergy it responds to a harmless substance as if it were a pathogen—a false alarm. The key point is sensitization: the first contact sensitizes the body, and visible reaction is produced by repeated exposure to the allergen. Sensitization itself does not yet mean disease: sensitivity can occur without symptoms, and both genes and environment contribute to the origin of allergy.

Pseudoallergy is distinguished separately: it occurs on the very first contact, without prior sensitization and without an immunological mechanism, but is outwardly almost indistinguishable from allergy, as is food intolerance caused by non-immunological reactions.

Types of allergy and allergic diseases

Typical allergens: pollen of trees and grasses, house dust mites, mold spores, animal hair and dander, foods such as peanuts, milk, and eggs, insect stings, and certain medications. By allergen, allergy is distinguished as pollen allergy (hay fever), food allergy, drug allergy, allergy to insect stings, to house dust mites, to animals, and to mold. The diseases also differ: allergic rhinitis, urticaria, bronchial asthma; allergy and asthma often coexist in one person.

Types of allergic reactions

In 1963, Philip Gell and Robin Coombs described four types of hypersensitivity reactions that damage tissues. The first, anaphylactic, is mediated by IgE and IgG; the second, cytotoxic, by IgG and IgM; the third, immune-complex, by complexes involving IgG, IgM, and IgA; the fourth, delayed, by sensitized lymphocytes.

In 2001, EAACI, and in 2004, WAO refined the framework: reactions are divided into non-allergic, without a proven immunological mechanism, and allergic.

Main symptoms of an allergic reaction

Symptoms appear within minutes or several hours later, and the set depends on the organ involved.

• nose and eyes: runny nose, sneezing, tearing, redness;
• airways: cough, wheezing, shortness of breath, pain or tenderness in the cheeks, eyes, or forehead;
• skin and mucous membranes: raised rash, itching and redness, swelling of the eyes, lips, oral cavity, or throat;
• digestive tract: diarrhea, cramps, bloating, nausea, vomiting.

An allergic reaction by itself is not a reason to call an ambulance; 103/112—if the following appear: swelling of the lips, tongue, throat, or oral cavity, rapid breathing or shortness of breath, a tight throat and difficulty swallowing, bluish, gray, or pale skin and lips, sudden confusion, drowsiness, or dizziness, fainting from which the person cannot be awakened, lethargy in a child. These are signs of anaphylaxis, which requires treatment in a hospital.

How allergy is diagnosed and when to see a doctor

The reason for consultation is suspected allergy in oneself or in a child. The doctor may prescribe testing or refer to an allergy office. Skin tests are used, when a small amount of allergen is applied to the skin and the reaction is assessed, blood tests, and in some cases gradual administration of increasing doses of the suspected product under supervision.

The specialist may create an individual plan describing how to manage the allergy; what is suitable in a specific case is decided by the doctor.

This material is for reference: it explains how the phenomenon works but does not replace consultation with a doctor—diagnosis and management tactics are discussed only with them.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer