The term allergy or hypersensitivity describes a complex group of immunological reactions which are characterized by heightened responsiveness to antigen to which the animal has been previously exposed. The outward manifestations are of an inflammatory nature and may be generalized or localized in a particular organ or system. The degree to which an animal is affected can vary from swelling of the injection site to difficulty breathing.
Other signs include muscle tremors, anxiety, salivation, bloat, nasal discharge, vocalization, edema, diarrhea and dilation of blood vessels. This dilation of blood vessels then drops blood pressure. Between the blood pressure change and the difficulty breathing, death can easily occur without immediate action.