Hypersensitivity I-IV

Hypersensitivity reactions are inappropriate and excessive responses of the immune system to normally harmless substances. They are classified into four main types:

  1. Type I: Immediate (or Anaphylactic) Hypersensitivity
    1. Mechanism: Mediated by IgE antibodies which, upon exposure to an allergen, trigger mast cells and basophils to release various mediators including histamine.
    2. Examples:
      1. Allergic asthma (response to pollen, dust mites, or animal dander).
      2. Anaphylactic reactions to bee stings or certain foods like peanuts.
      3. Allergic rhinitis (hay fever).
      4. Urticaria (hives) and angioedema.
  2. Type II: Cytotoxic (or Antibody-Mediated) Hypersensitivity
    1. Mechanism: Caused by IgG or IgM antibodies directed against target antigens on specific cells or tissues, leading to their destruction.
    2. Examples:
      1. Hemolytic reactions due to mismatched blood transfusions.
      2. Autoimmune hemolytic anemia.
      3. Idiopathic Thrombocytopaenic Purpura (ITP)
      4. Pemphigus vulgaris (an autoimmune blistering disorder).
      5. Goodpasture syndrome (autoantibodies against kidney and lung basement membranes).
  3. Type III: Immune Complex-Mediated Hypersensitivity
    1. Mechanism: Caused by the deposition of antigen-antibody complexes in various tissues, which then activate complement and attract inflammatory cells.
    2. Examples:
      1. Systemic lupus erythematosus (SLE).
      2. Rheumatoid arthritis.
      3. Serum sickness (a reaction to antiserum or certain drugs).
      4. Post-streptococcal glomerulonephritis (kidney inflammation following streptococcal infections).
  4. Type IV: Delayed (or Cell-Mediated) Hypersensitivity
    1. Mechanism: Mediated by T cells rather than antibodies. Upon re-exposure to an antigen, activated T cells release cytokines that recruit and activate macrophages and other cells, leading to tissue damage.
    2. Examples:
      1. Tuberculin skin test (Mantoux test) in individuals previously exposed to Mycobacterium tuberculosis.
      2. Contact dermatitis (e.g., from poison ivy or nickel).
      3. Graft-versus-host disease (GVHD) in transplant recipients.
      4. Some forms of chronic asthma and chronic allergic rhinitis.