The Immunological Process of Sensitization
Under normal conditions the human immune system can be extremely efficient at identifying and eliminating pathogens and toxins that are a danger to the body. Allergies arise when the immune system mistakenly identifies a harmless substance as a threat and becomes sensitized to the substance.
The process begins when the individual is first exposed to the allergen, i.e. a substance generating allergies. The allergen is taken up and broken down into smaller peptides by antigen-presenting cells (APCs) of the immune system. The resulting peptides are then presented by the APCs to T cells (T helper cell type 2 – Th2), which release cytokines that induce B cells to produce allergen-specific IgE. These IgE molecules then bind to high-affinity receptors on basophils and/or mast cells. This step is called sensitization.
Subsequent exposure of a sensitized individual to the same allergen induces cross-linking of the IgE molecules bound to mast cells and basophils in the airways, the gut and the skin, activating these cells and causing them to release mediators such as histamine, prostaglandins, and leukotrienes, resulting in asthma, gastro-intestinal problems or skin rashes. If the allergen cross-links to IgE bound to basophils in the blood, these mediators can cause allergic shock. This step, called the immediate hypersensitivity or early-phase reaction, occurs within minutes of re-exposure to the allergen and usually resolves within 1 hour. The released mediators affect the blood vessels, mucous glands and bronchial tubes of the patient and usually cause tissue swelling.
Depending on the degree of sensitization, symptoms may range from as benign as a runny nose or a skin rash through to extremely severe reactions like allergic or an anaphylactic shock or even death.
Numerous substances in every day life have the potential to act as allergens. Allergens may be inhaled (e.g. pollen, dander), come in direct contact with the skin or mucous membranes (e.g. latex, poison ivy, oak), be injected (e.g. insect venom) or be ingested (e.g. peanuts, eggs).