An allergic reaction is an exaggerated immune response to a harmless substance, known as an allergen. Common allergens include pollen, dust mites, food and insect stings. When exposed to an allergen, the immune system mistakenly identifies it as a threat, triggering a cascade of inflammatory responses that lead to various symptoms.
Pathophysiology
1. Initial Sensitisation
- Upon initial exposure to an allergen, the body produces specific antibodies called IgE. These IgE antibodies bind to mast cells and basophils, which are immune cells found throughout the body, especially in areas like the skin, mucous membranes and around blood vessels.
2. Triggering the Reaction
- During subsequent exposures to the same allergen, it binds to the IgE antibodies attached to mast cells. This binding triggers a process called mast cell degranulation, leading to the release of inflammatory mediators.
3. Inflammatory Cascade
- Released inflammatory mediators, including histamine, leukotrienes and cytokines, cause a range of effects, including:
- Blood vessel dilation: Leading to redness, swelling, and warmth.
- Smooth muscle contraction: Causing bronchoconstriction in the airways and wheezing.
- Increased mucus production: Leading to runny nose, congestion and watery eyes.
- Inflammation: Triggering pain, itching, and redness.
4. Severity and Symptoms
- The severity of an allergic reaction depends on several factors, including the type and amount of allergen, the individual’s sensitivity and their overall health. Symptoms can range from mild (e.g., sneezing, itchy eyes) to severe (e.g., anaphylaxis, a potentially life-threatening reaction).