An allergic reaction is the inappropriately strong response of the immune system to an environmental antigen such as dust mites, pet dander, pollen, or certain foods. An allergic reaction is stimulated by the contact of an allergen with a helper T cell. The helper T cell stimulates B lymphocytes. A simulated B lymphocyte differentiates into a plasma cell, which produces IgE antibodies. IgE antibodies attach to mast cells or basophils at IgE receptor sites.
The next time the person comes in contact with the same allergen, the mast cell or basophil will respond by secreting large amounts of histamine. Histamine causes an allergic reaction, leading to symptoms such as a runny nose and watery, itchy eyes.