Makindo Medical Notes"One small step for man, one large step for Makindo" |
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Hypersensitivity reactions are exaggerated or inappropriate immune responses that result in tissue damage. They are classified into four types based on their mechanisms and clinical manifestations:
Type | Mediators | Mechanism | Clinical Examples | Clinical Importance |
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Type I (Immediate or Anaphylactic) | IgE, Mast Cells | IgE binds to mast cells, releasing histamine and other mediators within minutes of exposure to an allergen. | Allergic asthma, hay fever, anaphylaxis, food allergies | Can cause life-threatening anaphylaxis, requiring immediate intervention (e.g., epinephrine). Common in allergic diseases. |
Type II (Cytotoxic) | IgG, IgM, Complement | Antibodies (IgG or IgM) target cells, leading to complement activation and cell destruction. | Hemolytic anaemia, Goodpasture’s syndrome, rheumatic fever | Important in transfusion reactions, autoimmune hemolytic diseases, and some drug-induced conditions. |
Type III (Immune Complex-Mediated) | Immune complexes (Ag-Ab) | Antigen-antibody complexes form and deposit in tissues, causing inflammation and tissue damage. | Systemic lupus erythematosus (SLE), post-streptococcal glomerulonephritis, rheumatoid arthritis | Clinically relevant in autoimmune diseases and post-infection syndromes. Immune complex deposition can cause chronic inflammation. |
Type IV (Delayed-Type) | T Cells | Antigen exposure leads to a T-cell mediated response, causing tissue damage after 24–72 hours. | Contact dermatitis, tuberculosis, graft-versus-host disease | Important for diagnosing tuberculosis (e.g., tuberculin skin test) and managing transplant rejection and autoimmune diseases. |