article_snippet: When someone comes into contact with something they are allergic to (an allergen), a group of cells in the body - mast cells - release a substance called histamine.
article_snippet: Histamine causes the tiny blood vessels in the tissues of the body to leak fluid, which in turn causes the tissues to swell.
article_snippet: Signs of a mild allergic reaction include: .
article_snippet: Signs of a severe reaction include (in addition to the above): .
article_snippet: A severe reaction is known as anaphylaxis.  