There are three stages to the allergic response: In the first stage, the immune system encounters the foreign substance and identifies it as an invader. It then primes the immune system to recognize this invader as an enemy that needs to be destroyed in future encounters. This stage is known as sensitization. The subsequent stages are mast cell activation, and prolonged immune activation.
STAGE 1 — SENSITIZATION
The first time an allergen meets the immune system, no allergic reaction occurs. Instead, the immune system prepares itself for future encounters with the allergen. Scavenger cells called macrophages surround and break up the invading allergen. The macrophages then display the allergen fragments on their cell walls to specialized white blood cells, called T lymphocytes, which are the main orchestrators of the body's immune reaction. The T cells secrete a signaling chemical called interleukin-4, which activates other white blood cells known as B lymphocytes. These cells secrete antibodies specific for that particular allergen. These antibodies, called immunoglobulin E (IgE) receptors, are attached to cells in the immune system, called mast cells and basophils. Individuals prone to allergies are known to have abnormally high levels of IgE antibodies.
STAGE 2 — MAST CELL ACTIVATION
Stage 2, or mast cell activation, represents a later encounter between the allergen and the immune system and usually occurs within minutes after the second exposure to an allergen. IgE antibodies on mast cells, constructed during the sensitization phase, recognize the allergen and bind to the invader. Once the allergen is bound to the receptor, granules in the mast cells release their contents.
These contents, or mediators, are substances such as histamine, platelet-activating factor, prostaglandins, and leukotrienes. Mediators are what actually trigger the allergy attack. Histamine stimulates mucus production and causes redness, swelling, and inflammation. Prostaglandins constrict airways and enlarge blood vessels.
STAGE 3 — LATE PHASE INFLAMMATORY RESPONSE
In Stage 3 tissue mast cells and neighboring cells produce chemical messengers that signal circulating basophils, eosinophils, and other cells to migrate into that tissue, to help fight the foreign material. These recruited immune cells secrete chemicals of their own that sustain inflammation, cause tissue damage, and recruit yet more immune cells. This phase occurs several hours after exposure and can last for hours and even days.