Immunology/Hypersensitivity

An allergic response, or hypersensitivity, is an immune system overreaction.

There are four kinds of hypersensitivity:

Immediate Hypersensitivity (Type I) - an immediate response to an allergen (a foreign substance that poses no danger in and of itself yet is treated as an antigen) that the immune system has had prior exposure to.
Cytotoxic Hypersensitivity (Type II) - an immune response to human cells with foreign antigens. Patients require blood type compatibility for transfusion because of the dangers of Type II hypersensitivity.
Immune Complex Hypersensitivity (Type III) - an immune response to immune complexes (antigen-antibody complexes) that become stuck in a given area. This results in continuous immune attack (and ensuing tissue damage).
Delayed Hypersensitivity (Type IV) - an immune response to foreign substances that occurs over the course of several hours. Poison Ivy's effects are the result of Type IV hypersensitivity.