Term used to describe an exaggerated response to a foreign substance (i.e., an antigen), which causes tissue damage in a host after repeated challenges, causing the individual to become sensitized. Type I hypersensitivity mediated by IgE antibodies and produces an immediate (minutes to an hour) reaction; e.g. peanut, latex, penicillin allergy. Type IV hypersensitivity is cell mediated and response requires 24-72 hours for the rash-like symptoms to fully express, skin transplants to be rejected, etc.