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How does type I hypersensitivity differ from type IV hypersensitivity?

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immune system disorder False-color scanning electron micrograph of a T cell infected with HIV (human immunodeficiency virus), the agent that causes AIDS (acquired immunodeficiency syndrome).

Type I and type IV hypersensitivities differ primarily in their underlying immune mechanisms and the speed of their onset.

Type I Hypersensitivity
* Mechanism: This is an antibody-mediated reaction involving immunoglobulin E (IgE) antibodies. These antibodies bind to mast cells and basophils, which release chemicals like histamine upon re-exposure to an allergen.12
* Onset: It is characterized by an immediate reaction, often occurring within minutes of contact with the allergen.32
* Examples: Common manifestations include hay fever, asthma, hives, and anaphylaxis.412

Type IV Hypersensitivity
* Mechanism: This is a cell-mediated reaction that does not involve antibodies. It relies on T cells that have been sensitized by prior exposure to an antigen. When restimulated, these T cells secrete cytokines that recruit and activate other immune cells to the site.521
* Onset: Known as delayed-type hypersensitivity, these reactions typically develop slowly, appearing 12 to 24 hours or more after exposure.512
* Examples: Common examples include contact dermatitis and the tuberculin test.516