Answer:
Here's a brief comparison of the four types of hypersensitivity reactions:
Type I Hypersensitivity:
Mediator: IgE antibodies
Antigen: Allergens (e.g., pollen, dust mites, certain foods)
Mechanism: IgE antibodies bind to mast cells and basophils, leading to release of histamine and other inflammatory mediators.
Examples: Allergic rhinitis, asthma, anaphylaxis
Type II Hypersensitivity:
Mediator: IgG or IgM antibodies
Antigen: Cell surface or tissue-specific antigens
Mechanism: Antibodies bind to antigens on cells, activating complement system or causing antibody-dependent cell-mediated cytotoxicity (ADCC).
Examples: Blood transfusion reactions, autoimmune hemolytic anemia
Type III Hypersensitivity:
Mediator: IgG or IgM immune complexes
Antigen: Soluble antigens (e.g., circulating immune complexes)
Mechanism: Immune complexes deposit in tissues, activating complement and attracting neutrophils, leading to inflammation and tissue damage.
Examples: Systemic lupus erythematosus, post-streptococcal glomerulonephritis
Type IV Hypersensitivity:
Mediator: T cells (CD4+ or CD8+)
Antigen: Typically, protein antigens or haptens that bind to self-proteins
Mechanism: Sensitized T cells release cytokines, activating macrophages or cytotoxic T cells, causing tissue damage.
Examples: Contact dermatitis (e.g., poison ivy), tuberculin skin test reaction