A hypersensitive immune response to a substance. An allergic reaction can occur when the immune system attacks a normally harmless substance. The allergic immune system calls upon a protective antibody called immunoglobulin E (IgE) to fight these invading sub- stances. In the melee, cells called mast cells release a variety of strong chemicals, including histamine, into the tissues and blood. This chemical release is irritating and causes itching and swelling and can also lead to lung airway narrowing and throat tight- ening, as is found in asthma, as well as to loss of voice. For example, this is how hay fever and aller- gic pink eye (conjunctivitis) occur.