reactions occur when our overreacts to harmless substances. These reactions can range from mild allergies to life-threatening , involving complex interactions between , , and immune cells.
There are four main types of reactions, each with unique mechanisms and symptoms. Understanding these types helps diagnose and treat allergies, autoimmune disorders, and other immune-mediated conditions effectively.
Types and Mechanisms of Hypersensitivity Reactions
Introduction to Hypersensitivity
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Hypersensitivity reactions occur when the immune system responds excessively or inappropriately to an antigen, leading to tissue damage and disease. These reactions involve complex interactions between allergens, antibodies, and various immune cells, often resulting in and other harmful effects.
Types of hypersensitivity reactions
Type I (Immediate) Hypersensitivity
Mediated by antibodies bound to and
cross-links IgE triggering and release of , , and other inflammatory mediators
Causes , smooth muscle contraction, and increased mucus secretion leading to symptoms (, , )
Involves a phase where initial exposure to an allergen leads to IgE production
Type II (Antibody-Mediated) Hypersensitivity
Mediated by and antibodies binding to cell surface antigens
Leads to cell destruction through activation and
Results in cell lysis, inflammation, and tissue damage (autoimmune hemolytic , Goodpasture syndrome)
Type III (-Mediated) Hypersensitivity
Antigen-antibody complexes deposit in tissues activating and recruiting immune cells
Causes inflammation and tissue damage (, systemic lupus erythematosus)
Type IV (Delayed-Type) Hypersensitivity
Cell-mediated immune response involving activated T cells releasing
Triggers inflammation and tissue damage with delayed onset of 24-72 hours after exposure
Occurs in and positive skin tests
Blood type incompatibility issues
Four main types determined by presence or absence of A and B antigens on : A, B, AB, and O
Natural antibodies form against missing antigens (anti-A in type B individuals, anti-B in type A)
Transfusion of mismatched blood leads to recipient antibodies attacking donor red blood cells causing agglutination and
Rh(D) antigen either present (Rh+) or absent (Rh-) on red blood cells
Rh- individuals can develop anti-Rh antibodies if exposed to Rh+ blood
Subsequent Rh+ exposure triggers rapid antibody response and destruction of Rh+ red blood cells
Occurs when Rh- mother carries Rh+ fetus and develops anti-Rh antibodies
Maternal anti-Rh antibodies cross placenta and attack fetal red blood cells
Leads to fetal anemia, jaundice, and potentially hydrops fetalis
Prevented by administering () to Rh- mothers
Diagnosis and treatment of hypersensitivities
Diagnosis: for allergens, measuring serum IgE levels, basophil activation tests
Treatment: allergen avoidance, antihistamines to block effects, for anaphylaxis
Type II Hypersensitivity
Diagnosis: direct and indirect for antibodies, tissue biopsy
Treatment: corticosteroids and immunosuppressants to reduce inflammation, to remove antibodies