Such a reaction may also occur from eating a certain food or taking a drug. The body reacts to an antigen by forming antibodies. The antibodies bind to the antigen, usually rendering it harmless. Sometimes, however, when the antibody and antigen interact, inflammation and tissue damage occur. Allergic reactions are classified by the type of tissue damage that develops.
Many allergic reactions are a mixture of more than one type of tissue damage.
When the antigen binds to these cell-surface antibodies, the mast cells release substances, such as histamine, that cause the blood vessels to dilate and the airways to narrow. These substances also attract other white blood cells to the area. An example of a type I reaction is allergic bronchial asthma. An example of a disease caused by a type II reaction is Goodpasture's syndrome. They may cause widespread inflammation that damages tissues, particularly blood vessel walls, a condition called vasculitis.