These ailments are all caused by allergic reactions to various substances. In most cases, an allergy causes relatively mild symptoms such as sneezes from breathing dust. A rare and severe allergic reaction can trigger anaphylactic shock that occasionally leads to death. Death from bee stings is an example of this severe type of allergic reaction. An allergy develops in much the same way as does an immunity against an infectious disease.
Several things happen to cause an allergy.
First, a person is exposed to a substance such as pollen. The bodies of most people simply ignore pollen, but others treat pollen as a foreign invader. When this happens, a substance in the blood known as an antigen becomes active. Antigens cause the formation of antibodies whose job it is to fight off the invading substance. Antibodies are normally in the blood and are activated by the antigen to which each is specifically linked.