1. (of a cell) The alteration of the integrity of a plasma membrane resulting from the reaction of specific antibodies with antigens on the surface of the cell. In the presence of complement, the cell ruptures.

2. (of an individual) Initial exposure to a specific antigen such that re-exposure to the same antigen causes a severe immune response, as in allergies or anaphylaxis.

3. (in animal behaviour) The process whereby an animal gives a stronger than normal response to a stimulus after previous application of a different stimulus. For example, if the tail of a sea slug (Aplysia) is given an electric shock, a subsequent tap on the animal’s siphon will cause a marked withdrawal of the siphon that is greater than that without a preceding tail shock. Compare dishabituation.