Hypersensitivity Reactions (Types I–IV)

Overview of Hypersensitivity

• Definition: exaggerated, excessive, and inappropriate immune response to normally harmless antigens (allergens)
• Consequences
  • Range from mild, localized irritation to severe, systemic, life-threatening reactions
  • Potential for extensive tissue damage, chronic disability, or death
• Nomenclature
  • Four distinct categories: $I$, $II$, $III$, $IV$ (always written in Roman numerals)
• Key distinguishing factors between types
  • Speed of onset (immediate vs. delayed)
  • Effector mechanism (antibody-mediated vs. T-cell-mediated)
  • Immunoglobulin class involved ( $IgE, IgG, IgM$ )
  • Complement participation
  • Local vs. systemic manifestations

General Pathophysiologic Themes

• Initial exposure → sensitization → memory response on re-exposure
• Possible cell participants
  • Mast cells, basophils, eosinophils
  • B-cells (antibody production)
  • T-helper (Th) and cytotoxic (Tc) lymphocytes
  • Macrophages / neutrophils (phagocytes)
  • Natural killer (NK) cells
• Principal chemical mediators
  • Histamine, vasoactive amines → vasodilation, ↑ capillary permeability, edema
  • Cytokines, lysosomal enzymes → inflammation, tissue necrosis
• Complement system (classical pathway) amplifies cell lysis and inflammation in Types $II$ and $III$

Type I Hypersensitivity (Immediate / $IgE$-Mediated)

• Hallmark: rapid onset minutes after re-exposure; most common type encountered clinically
• Mechanism of action
  • Allergen exposure → B-cell class-switch to $IgE$
  • Excess $IgE$ binds Fc receptors on mast cells & basophils (sensitization)
  • Subsequent exposure → cross-linking of bound $IgE$ → degranulation
  • Release of histamine, leukotrienes, prostaglandins, eosinophil chemotactic factors, etc.
• Arrow-style sequence (summarised)
  1. Initial allergen exposure (food, drug, pollen…)
  2. $IgE$ synthesis and mast-cell coating
  3. Re-exposure → allergen cross-links $IgE$
  4. Mast-cell/basophil degranulation
  5. Vasoactive mediators → vasodilation, ↑ permeability, smooth-muscle spasm
  6. Clinical manifestations (rash, bronchospasm, edema, anaphylaxis)
• Clinical examples
  • Atopic dermatitis, urticaria, allergic rhinitis (hay fever)
  • Allergic asthma (bronchoconstriction)
  • Food allergies (GI cramping, diarrhea)
  • Drug allergies (penicillin, sulfa) → possible systemic collapse & hypotension
  • Blood-borne environmental antigens
• Nursing / practical implications
  • Thorough allergy history pre-medication administration to avoid anaphylaxis
  • Emergency preparedness: epinephrine, airway management

Type II Hypersensitivity (Antibody-Mediated, Tissue-Specific)

• Also called "cytotoxic" hypersensitivity; often immediate
• Mediators: $IgG$ or $IgM$ + complement
• Key concept: "mistaken identity"—antibodies bind antigens present on specific cell surfaces or extracellular matrix
• Resulting effects
  • Complement activation → membrane attack complex → cell lysis
  • Opsonization → phagocytosis by macrophages
  • NK-cell-mediated cytotoxicity
  • Functional blockade of receptors (without cell death)
• Arrow-style sequence
  1. Antigen on cell membrane (self or exogenous)
  2. $IgG/IgM$ binds antigen
  3. Complement recruitment/activation
  4. Phagocytes & NK cells triggered
  5. Cell destruction or receptor dysfunction → clinical disease
• Clinical examples / conditions
  • Hemolytic anemia (RBC lysis) → fatigue, pallor, jaundice
  • Myasthenia gravis (antibody blocks acetylcholine receptor)
  • Type II diabetes (antibody to insulin receptors—less common autoimmune form)
  • Transfusion reactions, hemolytic disease of the newborn
• Long-term possibility: contributes to multiple autoimmune disorders

Type III Hypersensitivity (Immune-Complex-Mediated)

• Mediator: soluble $IgG$ (occasionally $IgM$) immune complexes
• Timing: subacute; can be chronic & systemic
• Mechanism
  • Antibody binds soluble antigen in blood/body fluids → antigen-antibody complex
  • Small/medium complexes evade phagocytosis → deposit in vessel walls, joints, kidneys, etc.
  • Complement activation → recruitment of neutrophils (polymorphonuclear leukocytes, PMNs)
  • Neutrophil degranulation → lysosomal enzymes → local tissue injury
• Arrow-style sequence
  1. Formation of soluble immune complexes in circulation
  2. Deposition in tissues (glomeruli, synovia, vessels)
  3. Complement + neutrophil activation
  4. Enzymatic tissue damage → inflammation
  5. Possible necrosis & organ dysfunction
• Clinical examples
  • Systemic lupus erythematosus (SLE)
  • Post-streptococcal glomerulonephritis
  • Serum sickness, drug-induced vasculitis
  • Farmer’s lung (hypersensitivity pneumonitis)
• Prognostic note
  • Initial exposure may resolve; repeated exposure intensifies response and can be fatal

Type IV Hypersensitivity (Cell-Mediated / Delayed)

• Mediated exclusively by T lymphocytes (no antibodies)
• Two principal T-cell subsets involved
  1. Helper T (Th₁) cells → release cytokines (IFN-γ, IL-2) → activate macrophages
  2. Cytotoxic T (Tc) cells → direct cell lysis via perforin/granzyme
• Timing: delayed onset ( $24–72$ h after exposure)
• Arrow-style mechanism
  1. Antigen penetrates skin/tissue
  2. APC presents antigen to naive T cell → differentiation to Th₁/Tc
  3. Memory T cells formed (sensitization)
  4. Re-exposure → rapid T-cell activation
  5. Cytokine storm & macrophage recruitment → granuloma or dermatitis
  6. Tissue destruction secondary to enzymatic release
• Clinical examples
  • Allergic contact dermatitis (latex, poison ivy, nitrile-sensitive individuals)
  • Tuberculin skin test (PPD) → indurated lesion indicates prior sensitization
  • Granulomatous diseases: tuberculosis, leprosy, sarcoidosis
  • Chronic graft rejection (transplanted organs/tissues)
• Occupational / nursing considerations
  • Routine assessment of glove sensitivity for staff & patients
  • Substitute nitrile or alternative barrier methods if dermatitis occurs

Comparative Snapshot (Quick Reference)

• Type I: $IgE$; mast-cell degranulation; minutes; edema, bronchoconstriction, anaphylaxis
• Type II: $IgG/IgM$ + complement; cell-bound antigen; minutes-hours; cytolysis or receptor blockade
• Type III: $IgG$ complexes; complement + neutrophils; hours-weeks; vasculitis, nephritis, arthritis
• Type IV: T cells; no antibody; >24 h; granulomas, contact dermatitis, transplant rejection

Clinical Management & Nursing Pearls

• Type I
  • Immediate: epinephrine $0.3–0.5\,\text{mg}$ IM, airway support, antihistamines, corticosteroids
  • Prevention: allergy testing, desensitization therapy, strict drug-allergy documentation
• Type II
  • Remove offending agent (drug, transfusion), immunosuppression (corticosteroids)
  • Plasmapheresis in severe autoimmune hemolysis
• Type III
  • Anti-inflammatory agents, immunosuppressants (e.g., cyclophosphamide in SLE)
  • Monitor renal function; manage hypertension
• Type IV
  • Topical/systemic steroids for dermatitis
  • Avoidance of contact allergens; immunosuppressive therapy for graft rejection
• Overarching ethical/practical considerations
  • Informed consent includes discussion of potential allergic reactions to medications or materials
  • Meticulous documentation & communication of known allergens among healthcare team

Key Takeaways & Exam Reminders

• Always write hypersensitivity categories in Roman numerals: $I, II, III, IV$
• Memorize antibody class & effector cells for each type (commonly tested)
• Link clinical vignette clues (e.g., wheal-and-flare within minutes) to the correct hypersensitivity mechanism
• Understand complement’s role: heavy in Types $II$ & $III$, absent in $I$ & $IV$
• Patient safety begins with allergy history—forgetting this step can lead to preventable anaphylaxis