Allergies are abnormal immune reactions to specific agents known as antigens or allergens.
Examples of common allergens are foods, drugs, pollens, dust mites, mold spore, animal danders, feathers, and insect venoms, as well as other substances which nonallergic people find relatively nondangerous with usual exposure. Thereafter, repeat exposure to such a substance may evoke an allergic response with the release of a histamine and a variety of pro- inflammatory substances, including enzymes, leukotrienes, and interleukins, and the creation of damaging free radicals. The multiplicity of these proinflammatory substances explains why single medications such as antihistamines often fail to completely control symptoms. Types of Reactions Allergic reactions may involve any part of the body, but they typically involve the body surfaces, which serve as exposure points and gateways. Headaches, dizziness, fatigue, joint and muscle pains, and many other symptoms can also be allergic in nature.