Published: 18 Jun 2025

ICD9: 995.2      ICD10: T78.40XA      ICD11: 4A8Z

Hypersensitivity refers to exaggerated or inappropriate immune responses that result in tissue damage or disease.
Essentially, it's when your immune system overreacts to a harmless substance or attacks your own body. Think of it as your immune system misidentifying a threat and launching a full-scale attack when only a mild response was needed, or no response at all.

Here's a breakdown:

Immune Response: Hypersensitivity involves the body's immune system, which normally protects against pathogens like bacteria and viruses.

Exaggerated or Inappropriate: The response is *out of proportion* to the threat posed by the antigen (the substance triggering the response), or the antigen is something that shouldn't trigger a response at all (like your own body's tissues).

Tissue Damage or Disease: This inappropriate response leads to inflammation and damage in the body, causing a variety of symptoms and diseases.

Different Types of Hypersensitivity (Gell and Coombs Classification):

Hypersensitivity reactions are classified into four main types, based on the mechanisms involved:

1. Type I (Immediate Hypersensitivity):
Mechanism: IgE antibody-mediated. IgE binds to mast cells and basophils. Subsequent exposure to the allergen causes these cells to release inflammatory mediators like histamine.
Examples: Allergies (hay fever, asthma, food allergies), anaphylaxis.
Timing: Reactions occur rapidly, within minutes to hours of exposure.

2. Type II (Antibody-Mediated Cytotoxic Hypersensitivity):
Mechanism: IgG or IgM antibodies bind to antigens on cell surfaces, leading to cell destruction by complement activation or antibody-dependent cell-mediated cytotoxicity (ADCC).
Examples: Hemolytic transfusion reactions, hemolytic disease of the newborn (Rh incompatibility), autoimmune hemolytic anemia.
Timing: Reactions can take hours to days to develop.

3. Type III (Immune Complex-Mediated Hypersensitivity):
Mechanism: IgG or IgM antibodies form immune complexes with soluble antigens. These complexes deposit in tissues, activating complement and causing inflammation.
Examples: Serum sickness, rheumatoid arthritis, systemic lupus erythematosus (SLE), glomerulonephritis.
Timing: Reactions take several hours to days to develop.

4. Type IV (Delayed-Type Hypersensitivity):
Mechanism: T cell-mediated. Sensitized T cells release cytokines upon exposure to the antigen, leading to inflammation and tissue damage. There are subtypes based on the specific T cell population involved (Th1, Th2, CTL).
Examples: Contact dermatitis (poison ivy), tuberculin reaction (TB skin test), graft rejection, some autoimmune diseases.
Timing: Reactions are delayed, typically taking 48-72 hours to develop.

Key differences between the types:

Antibody Involvement: Types I, II, and III involve antibodies (IgE, IgG, IgM), while Type IV is T cell-mediated.
Timing: Type I reactions are immediate, while Types II, III, and IV are delayed.
Mechanism of Damage: Each type uses a different mechanism to cause tissue damage.

In summary, hypersensitivity is an overzealous immune response that can lead to a variety of diseases. Understanding the different types of hypersensitivity is crucial for diagnosis and treatment.