1. A hypersensitivity reaction to the ingestion or injection of a substance (a protein or drug) resulting from a prior contact with a substance.
2. The hypersensitive state which may develop after the introduction of a foreign protein or other antigen into the body tissues.
3. A severe, immediate, potentially fatal bodily reaction to contact with a substance (antigen) to which the individual has previously been exposed.

It is often triggered by antiserum, antibiotics, or insect stings.