Hypersensitivity refers to an exaggerated or inappropriate immune response to a foreign substance, which may lead to tissue damage and various clinical symptoms. It is classified into four main types based on the mechanism involved and the time course of the reaction:
1. Type I - Immediate Hypersensitivity (Anaphylactic Reactions)
- Mechanism: Involves IgE antibodies binding to allergens and triggering the release of histamine and other mediators from mast cells and basophils.
- Common Triggers: Pollen, dust mites, animal dander, certain foods, insect venoms, and latex.
- Examples: Allergic rhinitis (hay fever), asthma, food allergies, and anaphylaxis.
- Clinical Features: Symptoms may include sneezing, itching, hives, bronchoconstriction, and in severe cases, anaphylactic shock.
2. Type II - Cytotoxic Hypersensitivity (Antibody-Mediated)
- Mechanism: Involves IgG or IgM antibodies that bind to antigens on surface cells, leading to cell destruction through complement activation or cytotoxic cells.
- Common Triggers: Drug reactions, incompatible blood transfusions, and autoimmune disorders.
- Examples: Hemolytic anemia, Goodpasture syndrome, and rheumatic fever.
- Clinical Features: Symptoms vary depending on the affected tissue, such as anemia, jaundice, or kidney damage.
3. Type III - Immune Complex-Mediated Hypersensitivity
- Mechanism: Characterized by the formation of immune complexes (antigen-antibody complexes) that deposit in tissues, leading to inflammation and tissue damage.
- Common Triggers: Persistent infections, autoimmune diseases, and certain environmental antigens.
- Examples: Systemic lupus erythematosus, rheumatoid arthritis, and serum sickness.
- Clinical Features: Symptoms can include fever, rash, polyarthritis, and glomerulonephritis, depending on where the immune complexes accumulate.
4. Type IV - Delayed-Type Hypersensitivity (Cell-Mediated)
- Mechanism: Involves T cell-mediated responses instead of antibodies. Sensitized T cells respond to an antigen upon re-exposure, leading to inflammation and tissue damage.
- Common Triggers: Various antigens, including metals, certain drugs, and infectious agents.
- Examples: Contact dermatitis (e.g., poison ivy), tuberculosis skin test (Mantoux test), and graft-versus-host disease.
- Clinical Features: Symptoms usually appear hours to days after exposure and can include redness, swelling, and itching at the site of contact.
General Considerations
- Diagnosis: Hypersensitivity reactions can be diagnosed through patient history, skin tests, blood tests (such as specific IgE levels), and patch tests, depending on the type of hypersensitivity.
- Treatment: Management strategies may include avoiding known allergens, using antihistamines, corticosteroids, or immunotherapy, and in severe cases, employing epinephrine injections for anaphylactic reactions.
- Prevention: Awareness of triggers, education about the condition, and proper medical management can help prevent hypersensitivity reactions.
Understanding hypersensitivity is crucial for managing allergic and autoimmune diseases, as inappropriate immune responses can lead to significant morbidity and impact quality of life.
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