Hypersensitivity

Overview of Innate and Adaptive Immunity

  • Innate immunity: normal body defense mechanisms

  • Adaptive immunity: recognition and response to threats

Innate Immunity

  • First Line of Defense: Barriers against pathogens

    • Skin and mucous membranes

  • Second Line of Defense: Internal defenses

    • Phagocytic cells, inflammation, fever

  • Inflammation: Body's response to injury

    • Acute vs. chronic inflammation

  • Wound Healing: Repair mechanisms of the body

Adaptive Immunity

  • Immune system evolution from neonate to geriatric stages

  • Recognition of specific threats

  • Importance of prior knowledge from pathophysiology courses

Alterations in Immune and Inflammatory Responses

  • Exaggerated immune reactions

  • Misdirected immune actions (Autoimmunity)

  • Poor immune response (Immunodeficiency)

Hypersensitivity Reactions

  • Definition: Exaggerated immune response causing damage to the host

  • Sensitization: Initial exposure that leads to possible reactions upon re-exposure

  • Four Classifications of Hypersensitivity Reactions:

    • Type I: Immediate

    • Type II: Tissue-specific

    • Type III: Immune complex mediated

    • Type IV: Delayed hypersensitivity

Type I Hypersensitivity (IgE Mediated)

  • Definition: Exaggerated response to environmental antigens (allergens)

  • Examples: Hay fever, asthma, eczema, food allergies

  • Mechanism:

    • IgE binds to mast cells, leading to degranulation and release of histamine

    • Symptoms: Increased vascular permeability, bronchial smooth muscle contraction, gastric acid secretion

  • Anaphylaxis: Most severe complication; life-threatening

    • Symptoms include itching, erythema, systemic response with vasodilation, and respiratory distress

    • Common triggers: Bee stings, peanuts, eggs, shellfish

Type II Hypersensitivity (Tissue-Specific)

  • Definition: Reaction to specific tissues due to antibodies against antigens on cell membranes

  • Examples:

    • Autoimmune thrombocytopenia (platelets)

    • Graves' disease (thyroid stimulating hormone receptor)

    • Goodpasture syndrome (kidney membrane destruction)

    • Transfusion reactions due to ABO mismatching

  • Mechanism:

    • Cell lysis via membrane attack complexes formed by antibodies and complement activation

    • Consequences include hemolytic anemia and organ dysfunction

Type III Hypersensitivity (Immune Complex Mediated)

  • Definition: Immune complexes deposit in tissues

  • Examples: Serum sickness, Raynaud's phenomenon, lupus, rheumatoid arthritis

  • Mechanism:

    • Antigen-antibody complexes activate complement, resulting in localized inflammation

    • Symptoms vary based on affected tissues: arthritis, vasculitis, glomerulonephritis

Type IV Hypersensitivity (Delayed)

  • Definition: T-cell mediated responses that take days to manifest

  • Examples: Contact dermatitis, Steven Johnson syndrome, drug reactions

  • Mechanism:

    • Activation of T cells and macrophages leads to tissue damage

    • Granulomatous responses can occur in conditions like tuberculosis and sarcoidosis

Disorders Related to Hypersensitivity

  • Allergies: hypersensitivity to environmental antigens

    • Genetic predisposition must be considered

    • Symptoms vary but can include conjunctivitis, urticaria, asthma

    • Testing includes skin tests and measuring IgE levels

    • Desensitization can reduce life-threatening reactions

  • Autoimmunity:

    • Breakdown of immune tolerance

    • Examples: lupus, MS, celiac disease, myasthenia gravis

    • Increased prevalence in females; family history often involved

  • Alloimmunity: Immune response against tissues from another individual

    • Examples: Transplant rejection, blood transfusion reactions

    • Transfusion reactions can be immediate or delayed

Transfusion and Transplant Reaction Mechanisms

  • ABO System:

    • A and B antigens determine blood types; universal donors and recipients

    • IgM antibodies responsible for most transfusion reactions

  • Rh Factor:

    • Rh positive and negative categorization based on D antigen

    • Hemolytic disease of the newborn: Rh incompatibility leading to anemia in infants

  • Transplant Rejection:

    • Can be hyperacute (minutes), acute (days to months), or chronic (months to years)

    • Managed through HLA matching and immunosuppressive therapy to prevent rejection reactions

Autoimmunity Example: Systemic Lupus Erythematosus (SLE)

  • Characteristics: Production of autoantibodies against a range of self-antigens

  • Symptoms: Skin rashes, joint pain, organ involvement (kidneys), potential for renal failure

  • Management: Focus on minimizing systemic effects; treatment can include anti-inflammatories, steroids, and immunosuppressants

Conclusion

  • Understanding hypersensitivity reactions is crucial for clinical practice

  • Early recognition of symptoms and appropriate interventions can prevent severe outcomes

  • Collaboration across healthcare disciplines enhances patient safety and care in allergic, autoimmune, and alloimmune conditions.