Allergic reactions

General allergic reactions: What happens in the body?

An allergic reaction is an excessive, "mistaken" response of the immune system to a substance (allergen) that is completely safe for most people. Essentially, it is a failure in the "self‑nonself" recognition system, where the immune system starts attacking harmless proteins from food, pollen, or animal fur as if they were a dangerous virus or bacteria. Understanding this fundamental mechanism helps realize that allergy is not a sign of a "weak" immune system, but rather of its overactivity.

Mechanism of allergic reaction: Role of the immune system and histamine

The basis of most allergic reactions is a complex immune cascade, with key players being IgE class antibodies and special cells — mast cells.

1. Sensitization (first contact): Upon the first entry of an allergen into the body of an allergy‑prone person, the immune system produces specific IgE antibodies to that allergen. These antibodies "settle" on the surface of mast cells, which are abundant in the skin, respiratory tract, and gastrointestinal tract. This stage is asymptomatic.
2. Allergic reaction (subsequent contact): When the allergen enters the body a second time, it binds to the IgE antibodies on mast cells. This signals the mast cells to immediately release a large amount of biologically active substances into the surrounding tissues, the main one being histamine.

Histamine is responsible for most allergy symptoms: it dilates blood vessels (causing redness and swelling), increases their permeability, stimulates mucus production (causing runny nose) and irritates nerve endings (causing itching).

Types of allergic reactions: From skin to anaphylaxis

Depending on where the histamine release occurs, symptoms can vary:

• Skin: Hives (welts), angioedema (Quincke's edema), eczema flare‑ups.
• Respiratory: Allergic rhinitis (runny nose, sneezing), allergic conjunctivitis (itching and redness of the eyes), cough, wheezing, shortness of breath.
• Gastrointestinal: Nausea, vomiting, abdominal pain, diarrhea.
• Systemic (anaphylaxis): The most severe reaction, affecting multiple body systems simultaneously and threatening life.

Allergens can be a wide variety of substances, described in detail in articles about food allergy, seasonal allergies and pet allergy.