Hypersensitivity reactions are when the body’s immune system responds to something in an exaggerated or inappropriate way. As a result of this response, the body will come to harm. Hypersensitivity reactions include both allergic responses (attacks on non-self antigens) and autoimmune responses (attacks on self-antigens that are no longer recognised as self).

Typically, these reactions occur on the second exposure to an antigen once the body has already been primed to it.