How do allergies happen?

The first time the body encounters an allergen (foreign particle which causes an allergic reaction), plasma cells release IgE specific to that allergen. These plasma cells are stimulated by TH2 cells, which are a type of T helper lymphocytes. This IgE attaches to the surface of mast cells. This process makes the mast cells ready to react the next time the allergen is encountered.

When the allergen is encountered again, these mast cells are activated and release a chemical known as histamine, in addition to other chemicals involved in inflammation and allergic reactions. These chemicals cause the symptoms of allergies by reacting with parts of the body. This is the acute response, and typically happens within 5 – 10 minutes.

Histamine can cause airways to tighten (a process known as bronchoconstriction) such as in asthma; or can cause blood vessels to become more permeable, leading to fluid leaking (oedema) which causes hives (urticaria).

In addition, mast cells produce a group of chemicals called eicosanoids when they are activated by allergens. Eicosanoids consist of two main types of chemical – leukotrienes, and prostaglandins. These are chemicals derived from fatty acids (from fats) in the diet. Leukotrienes can cause prolonged airway constriction and can also cause other immune cells to arrive and act. They can also cause too much mucus to be created (mucus hypersecretion) – which can be seen in hay fever. Prostaglandins have similar effects, especially in relation to airway tightening.

In the late phase response, other cells become involved. Many of the chemicals that mast cells release can be used to draw in other cells of the immune system, especially lymphocytes, neutrophils, and eosinophils. This usually happens 4 – 6 hours after initial contact with the allergen.

Allergies Guide Index:

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