Type I–IV Hypersensitivity Reactions

Type I Hypersensitivity: Immediate, IgE-Mediated Reaction

• Sensitization Phase: Initial exposure to allergen (environmental antigen, food protein, drug hapten) triggers presentation to naive CD4+ T helper cells (Th2 differentiation) via antigen-presenting cells (dendritic cells, macrophages) in the context of MHC class II molecules. IL-4 and IL-5 secretion by Th2 cells promotes B cell class switching to IgE production. IgE antibodies bind with high affinity to FcεRI receptors on mast cells (tissue-resident) and basophils (circulating), a process termed "sensitization." Genetic predisposition (atopy) involves polymorphisms in IL-4 receptor-α, IgE receptor chains, and T cell receptor genes that lower the threshold for Th2 differentiation. Environmental factors (allergen dose, timing, route of exposure, concurrent infections, diesel exhaust particles) modulate this initial response.

• Re-exposure/Immediate Phase (Minutes): Upon re-exposure to the same allergen, cross-linking of FcεRI-bound IgE molecules occurs when divalent allergen bridges two adjacent IgE molecules. This clustering triggers rapid degranulation of mast cell and basophil granules, releasing preformed mediators: histamine (binds H1/H2 receptors causing vasodilation, increased vascular permeability, smooth muscle contraction, pruritus), tryptase (serine protease activating PAR-2 receptors), heparin (anticoagulant), and chondroitin sulfate. Simultaneously, de novo synthesis of lipid mediators occurs through arachidonic acid metabolism: prostaglandin D2 (PGD2; vasodilator, platelet aggregation), leukotriene C4 (LTC4) and its metabolites LTD4 and LTE4 (cysteinyl leukotrienes; potent smooth muscle constrictors, mucus secretion, eosinophil recruitment), and leukotriene B4 (LTB4; neutrophil chemotaxis). These mediators collectively produce urticaria, angioedema, bronchoconstriction, hypotension, and anaphylaxis within minutes.

• Late Phase (4–12 hours): Activated mast cells and basophils release cytokines (IL-4, IL-5, TNF-α, GM-CSF) that recruit eosinophils, neutrophils, and Th2 lymphocytes to tissue. Eosinophil infiltration is mediated by eotaxin (CCL11) and other chemokines. Eosinophils release major basic protein (MBP), eosinophil peroxidase (EPO), and eosinophil-derived neurotoxin (EDN) causing tissue damage and prolonged inflammation. This explains why symptoms may worsen 4–12 hours after initial reaction.

Type II Hypersensitivity: Cytotoxic, Antibody-Mediated Reaction

• Antibody Binding and Complement Activation: IgG or IgM antibodies bind to cell surface antigens (native antigens on red blood cells, platelets, neutrophils, or drug haptens) or tissue antigens. IgM and IgG1/IgG3 subclasses activate classical complement pathway through C1q binding; subsequent C3 activation generates C3b (opsonin for phagocytosis) and C5a (potent anaphylatoxin recruiting neutrophils/macrophages). This leads to complement-mediated cellular destruction either directly (C5b-9 membrane attack complex) or indirectly through complement-dependent cellular cytotoxicity (CDCC). Examples include drug-induced hemolytic anemia (penicillin G), thrombocytopenia (quinine), or neutropenia (sulfonamides).

• Antibody-Dependent Cellular Cytotoxicity (ADCC): IgG-coated target cells are recognized by FcγRIII receptors on natural killer (NK) cells, macrophages, and neutrophils. Receptor cross-linking activates these effector cells, triggering perforin/granzyme-mediated apoptosis and release of pro-inflammatory cytokines. This mechanism is central to Graves' disease (stimulating TSH-receptor antibodies) and myasthenia gravis (antibodies blocking acetylcholine receptors).

• Antibody-Mediated Inflammation: IgG binding to cell surface or tissue antigens without complement/ADCC can still induce local inflammation through FcγR signaling on resident macrophages and dendritic cells, activating them to release TNF-α, IL-6, and other pro-inflammatory mediators.

Type III Hypersensitivity: Immune Complex-Mediated Reaction

• Immune Complex Formation: When antigens are in slight antigen excess (compared to antibody), small-to-medium sized antigen-antibody complexes form rather than precipitate. These escape hepatic and splenic clearance by resident macrophages. Complexes deposit in tissues—classically in blood vessel walls (arteries and capillaries), glomerular basement membrane, and joints—where they activate complement and FcγRs.

• Complement and Cellular Activation: Deposited complexes activate classical complement pathway, generating C3a and C5a (anaphylatoxins recruiting neutrophils and macrophages). The C5b-9 membrane attack complex causes local vascular injury. Neutrophil infiltration releases proteolytic enzymes (elastase, collagenase) and reactive oxygen species, amplifying tissue destruction. IgG-containing complexes cross-link FcγRIIA on neutrophils, further amplifying inflammation.

• Temporal Evolution: Complexes form during the immune response phase (antibodies against persistent antigens), explaining why Type III reactions typically develop 3–8 weeks after exposure. Classic examples include serum sickness (after antitoxins, monoclonal antibodies, antibiotics), poststreptococcal glomerulonephritis, and systemic lupus erythematosus (SLE) with circulating anti-DNA complexes.

Type IV Hypersensitivity: Delayed, T Cell-Mediated Reaction

• Sensitization via Dendritic Cells: Hapten-protein conjugates or intact protein antigens are internalized and processed by dendritic cells via MHC class I (CD8+ CTL activation) and MHC class II (CD4+ Th1 activation) pathways. Unlike Type I reactions, no antibodies are generated; the reaction depends on clonal expansion of antigen-specific T lymphocytes over 24–72 hours (hence "delayed"). Th1 cells secrete IFN-γ and TNF-α; CD8+ CTLs produce perforin/granzyme.

• Effector Phase (24–72 hours after exposure): Upon re-exposure, tissue-resident memory T cells and infiltrating T cells recognize antigen presented on MHC molecules of macrophages, dendritic cells, or keratinocytes. CD4+ Th1 cells activate macrophages through IFN-γ and TNF-α, inducing them to release TNF-α, IL-1, IL-6, and macrophage migration inhibition factor (MIF), causing local inflammation and granuloma formation. CD8+ cytotoxic T lymphocytes (CTLs) directly kill infected or antigen-bearing cells through perforin/granzyme and Fas-FasL interactions. Keratinocytes expressing antigen undergo apoptosis, explaining epidermal necrosis in drug reactions and contact dermatitis.

• Chronic Inflammation: Continuous antigen presentation drives sustained T cell activation, leading to chronic inflammatory infiltration (lymphocytes, macrophages, epithelioid cells) and granuloma formation in mycobacterial infections, fungal infections, and berylliosis. This is mechanistically distinct from acute Type I inflammation.

Integration: How Pathophysiology Drives Clinical Manifestations

• Type I Rapidity and Anaphylaxis: IgE-mediated degranulation occurs within seconds-to-minutes because mast cells are tissue-resident and pre-armed with IgE, explaining the rapid onset of urticaria, angioedema, and anaphylaxis and the necessity for immediate epinephrine.

• Type II Tissue Specificity: The location of target antigens determines clinical presentation (hemolytic anemia if RBC antigens, thrombocytopenia if platelet antigens, thyroiditis if thyroid peroxidase antigens).

• Type III Vascular Tropism: Small immune complexes preferentially deposit in small vessels and glomeruli due to vascular permeability and hemodynamic factors, explaining vasculitis, arthritis, and glomerulonephritis.

• Type IV Chronicity: The requirement for T cell clonal expansion and cytokine-mediated macrophage activation explains the 24–72 hour delay and the potential for chronic persistence (e.g., tuberculin skin test).

TYPE I HYPERSENSITIVITY

• Environmental Allergens: Aeroallergens (pollens from grasses, trees, weeds; dust mite fecal particles containing Der p 1, Der p 2; animal dander proteins like Fel d 1 from cats; mold spores) are the most common triggers for allergic rhinitis and asthma in atopic individuals. Indoor allergens (dust mite, pet dander) are perennial; outdoor allergens show seasonal variation. Aeroallergen sensitization typically occurs in childhood, with highest prevalence in temperate climates with high pollen counts.

• Foods: Peanuts, tree nuts, shellfish, fish, eggs, milk, soy, wheat account for >90% of IgE-mediated food allergies. The prevalence of food allergy is approximately 2–3% in adults and 5–8% in children, with peanut allergy affecting 1–2% of children in developed countries. Cross-reactivity occurs between structurally similar proteins (e.g., birch pollen proteins and certain raw fruits in oral allergy syndrome). Food-dependent, exercise-induced anaphylaxis occurs when food is consumed within 2–3 hours of exercise.

• Medications: β-lactam antibiotics (penicillins, cephalosporins via hapten-protein conjugation), NSAIDs (particularly in patients with asthma/urticaria), ACE inhibitors, statins, vancomycin (direct mast cell degranulation—"red man syndrome"), and radiocontrast agents are common culprits. Sulfonamides can cause both IgE-mediated reactions and distinct Type IV reactions. The incidence of penicillin IgE-mediated reactions is approximately 0.1%, but cross-reactivity with cephalosporins is now recognized as <2% (lower than historically thought).

• Venoms: Hymenoptera venoms (bee, wasp, hornet) trigger IgE responses in 0.3–3% of the population; in beekeepers and farmers, sensitization rates exceed 15%. Fatal anaphylaxis from insect stings occurs in approximately 40–100 deaths/year in the United States.

• Latex: Latex proteins (Hev b proteins) from rubber gloves and medical equipment cause occupational allergies in 5–10% of healthcare workers; true IgE-mediated reactions occur in 0.4–1%. Cross-reactivity with avocado, banana, kiwi (due to homologous proteins) causes oral allergy syndrome.

• Atopy/Genetic Predisposition: Atopic individuals (20–30% of the population in developed countries) have elevated serum IgE levels and increased Th2 skewing. Polymorphisms in IL-4 receptor-α (IL4RA), FCER1β (FcεRI β-chain), and STAT6 genes increase susceptibility. Family history of atopy increases risk 2–3-fold; identical twins show >80% concordance.

TYPE II HYPERSENSITIVITY

• Drug-Induced Reactions: Penicillin and cephalosporins undergo metabolic degradation to form hapten-protein conjugates, most commonly with RBC membranes, causing drug-induced hemolytic anemia. Quinine and quinidine form complexes with platelets, triggering immune thrombocytopenia. Sulfonamides, allopurinol, anticonvulsants cause drug-induced neutropenia through similar mechanisms. The incidence of drug-induced hemolytic anemia is approximately 0.2–1 per million.

• Transfusion Reactions: ABO incompatibility (incompatible blood transfusion) and minor blood group incompatibilities (Rh, Kidd, Duffy) cause acute hemolytic transfusion reactions via IgG or IgM antibodies (Type II). Alloimmunization occurs in 1–2% of transfused patients.

• Autoimmune Hemolytic Anemia (AIHA): Idiopathic (most common; 50% of cases) or secondary to lymphoproliferative disorders (CLL, SLL), autoimmune diseases (SLE, antiphospholipid syndrome), or medications (methyldopa, quinidine). Cold agglutinin disease involves IgM antibodies against RBC I/i antigens, exacerbated by cold exposure.

• Graves' Disease: Autoimmune thyroiditis caused by IgG antibodies against TSH receptor, acting as agonists (stimulating hormone synthesis and secretion) rather than blocking antibodies.

• Myasthenia Gravis: IgG antibodies against nicotinic acetylcholine receptor at the neuromuscular junction in ~80% of generalized cases; muscle-specific kinase (MuSK) antibodies in seronegative cases. ADCC and complement activation result in neuromuscular junction destruction.

• Thrombotic Thrombocytopenic Purpura (TTP) and HIT: HIT (heparin-induced thrombocytopenia) caused by IgG antibodies against platelet factor 4 (PF4)-heparin complexes; ELISA sensitivities ~88%, specificities ~92%. TTP may involve Type II mechanisms in some cases (e.g., ticlopidine-induced TTP).

TYPE III HYPERSENSITIVITY

• Serum Sickness: Foreign serum or antitoxins (antivenom, antitoxins against diphtheria or tetanus), monoclonal antibodies (anti-TNF agents like infliximab, rituximab), beta-lactam antibiotics (especially amoxicillin), and NSAIDs form antigen-antibody complexes during the immune response phase. Incidence with monoclonal antibodies is approximately 5–10%; with amoxicillin, 1–3%.

• **Poststreptococcal Glomeruloneph

Type I (minutes after re-exposure)

• Urticaria and angioedema: histamine acting on H1 receptors of postcapillary venules produces wheals with surrounding flare and non-pitting, non-pruritic deep swelling of lips, tongue, and periorbital tissue. Stem cue: atopic child or young adult minutes after peanut, shellfish, or a bee sting.
• Bronchospasm and laryngeal edema: cysteinyl leukotrienes (LTC4/D4/E4) and histamine cause wheeze, stridor, hoarseness, and a sensation of throat closing — the airway findings that define impending death.
• Distributive shock: vasodilation and capillary leak produce hypotension, tachycardia, flushing, and syncope; GI smooth muscle contraction adds vomiting, cramping, and diarrhea.
• Chronic atopic phenotype: sneezing, clear rhinorrhea, boggy pale nasal turbinates, allergic shiners, and cobblestoned conjunctiva in seasonal or perennial rhinitis; late-phase eosinophilic inflammation explains symptom rebound hours later.

Type II (organ-specific, antibody-directed)

• Hemolysis: pallor, jaundice, scleral icterus, splenomegaly, dark urine — antibody-coated RBCs cleared by splenic macrophages or lysed by complement.
• Cytopenia syndromes: petechiae and mucosal bleeding in immune thrombocytopenia; new venous or arterial thrombosis with a falling platelet count 5–10 days after starting heparin in HIT.
• Receptor-directed disease: fatigable ptosis and diplopia (myasthenia gravis), heat intolerance and lid lag (Graves), hemoptysis with hematuria (anti-GBM, linear deposition).

Type III (days to weeks after antigen)

• Serum sickness triad: fever, urticarial or palpable purpuric rash, and symmetric polyarthralgia after antivenom, a chimeric monoclonal antibody, or a beta-lactam.
• Nephritic presentation: tea-colored urine, periorbital edema, and hypertension in poststreptococcal glomerulonephritis following pharyngitis or impetigo.

Type IV (24–72 hours)

• Contact dermatitis: linear vesicular streaks after poison ivy, nickel dermatitis under jewelry, or a rash conforming to a glove or shoe margin.
• Tuberculin induration, granulomatous disease, and drug eruptions ranging from morbilliform rash to targetoid lesions with mucosal erosions and Nikolsky sign in SJS/TEN.

Type I

• Anaphylaxis is a clinical diagnosis: the NIAID/FAAN criteria (endorsed in the AAAAI/ACAAI Joint Task Force anaphylaxis practice parameter) require acute onset with skin/mucosal involvement plus respiratory compromise or hypotension, or two organ systems after a likely allergen, or hypotension after a known allergen. Do not delay treatment for testing.
• Serum tryptase: drawn within the first few hours and compared with a convalescent baseline; a rise supports mast cell activation but a normal value does not exclude anaphylaxis (often normal in food-triggered events). Persistently elevated baseline tryptase suggests systemic mastocytosis or hereditary alpha-tryptasemia.
• Skin prick testing is the initial allergen-specific test (wheal compared with histamine and saline controls); serum allergen-specific IgE (ImmunoCAP) is used when skin is unavailable or antihistamines cannot be stopped. Total IgE is not diagnostic. Supervised oral food challenge remains the gold standard for food allergy, and graded drug challenge is the gold standard for drug allergy.

Type II

• Direct antiglobulin (Coombs) test is the key study: anti-IgG pattern indicates warm autoimmune hemolysis, anti-C3 pattern with a high-titer cold agglutinin and IgM anti-I indicates cold agglutinin disease. Supporting labs: elevated indirect bilirubin and LDH, low haptoglobin, reticulocytosis, spherocytes on smear.
• HIT: apply the 4Ts score first (ASH guideline-endorsed); intermediate/high probability prompts anti-PF4/heparin immunoassay, confirmed by a functional serotonin release assay.
• Autoantibody panels: acetylcholine receptor/MuSK antibodies, TSH-receptor antibodies, anti-GBM with linear IgG on immunofluorescence.

Type III

• Hypocomplementemia (low C3, low C4), elevated ESR/CRP, and circulating immune complexes; skin biopsy shows leukocytoclastic vasculitis with granular immunofluorescence. Renal biopsy in glomerulonephritis shows granular "lumpy-bumpy" deposits and subepithelial humps. ACR/EULAR classification criteria apply to SLE.

Type IV

• Patch testing is the gold standard for allergic contact dermatitis, read at roughly 48 and 96 hours. Tuberculin skin test induration is read at 48–72 hours using 5/10/15 mm thresholds by risk stratum, or replaced by an interferon-gamma release assay per CDC/ATS/IDSA guidance.

Immediate stabilization of anaphylaxis (Type I)

• Epinephrine, IM: first-line and life-saving per the AAAAI/ACAAI Joint Task Force and WAO — 0.3 mg IM (0.01 mg/kg, max 0.3–0.5 mg) into the anterolateral thigh, repeated every 5–15 minutes as needed. Alpha-1 vasoconstriction reverses hypotension and mucosal edema; beta-2 activity relieves bronchospasm; beta-1 supports inotropy.
• Adjuncts: supine positioning with legs elevated (sitting a hypotensive patient up risks empty ventricle syndrome), high-flow oxygen, large-volume isotonic crystalloid, and early airway assessment for intubation.
• Escalation: IV epinephrine infusion for refractory shock, inhaled beta-2 agonist (albuterol) for persistent wheeze, and glucagon when beta-blockade blunts the epinephrine response. H1 antihistamines (diphenhydramine, cetirizine) and glucocorticoids treat cutaneous symptoms only and must never substitute for or delay epinephrine.
• Definitive/long-term: allergen avoidance, prescribed epinephrine autoinjectors with an action plan, venom immunotherapy (highly effective for Hymenoptera anaphylaxis), oral immunotherapy for peanut, and anti-IgE therapy (omalizumab) in selected patients. There is no absolute contraindication to epinephrine in anaphylaxis.

Type II

• Withdraw the culprit drug first. Warm autoimmune hemolytic anemia: systemic corticosteroids (prednisone) first line, then anti-CD20 monoclonal antibody (rituximab) or splenectomy. Cold agglutinin disease responds poorly to steroids — cold avoidance and rituximab-based therapy are preferred.
• HIT (ASH guidelines): stop all heparin, start a non-heparin anticoagulant (argatroban, bivalirudin, or a direct oral anticoagulant). Warfarin is contraindicated until platelet recovery (venous limb gangrene), and prophylactic platelet transfusion is avoided.

Type I

• Biphasic anaphylaxis (emergency): late-phase mediator release causes recurrence hours after apparent resolution, which is why guideline-based observation after treatment is required; heralded by returning urticaria, wheeze, or falling blood pressure.
• Asphyxia from laryngeal edema and cardiovascular collapse (emergency): the two mechanisms of death in anaphylaxis; hoarseness or stridor mandates immediate airway control rather than watchful waiting.
• Airway remodeling in chronic asthma from sustained eosinophilic inflammation, and anaphylaxis-related myocardial ischemia — either from hypotension or from epinephrine-induced increases in myocardial oxygen demand; signaled by chest pain and ischemic ECG changes.
• Treatment harms: epinephrine causes tremor, palpitations, and hypertension (severe with inadvertent IV bolus dosing); first-generation antihistamines cause sedation and anticholinergic delirium in elderly patients.

Type II

• Severe anemia with high-output failure and cholelithiasis from chronic hemolysis (pigment stones); kernicterus in hemolytic disease of the fetus and newborn (emergency) when unconjugated bilirubin crosses the immature blood-brain barrier.
• HIT-associated thrombosis (emergency): platelet activation and thrombin generation produce limb-threatening arterial thrombosis, DVT/PE, or skin necrosis at injection sites; warfarin started too early causes venous limb gangrene.
• Myasthenic crisis (emergency): respiratory muscle weakness with falling negative inspiratory force requires ventilatory support, not simply more anticholinesterase.
• Treatment harms: corticosteroid-induced hyperglycemia, osteoporosis, and adrenal suppression; rituximab-associated hypogammaglobulinemia and hepatitis B reactivation; encapsulated-organism sepsis after splenectomy.

Type III

• Rapidly progressive glomerulonephritis with crescents (emergency): rising creatinine, oliguria, and active urinary sediment. Chronic immune-complex nephritis progresses to CKD, staged and monitored per KDIGO.
• Vasculitic tissue infarction and persistent serum sickness on continued antigen exposure.

Type IV

• Secondary bacterial infection and lichenification of excoriated contact dermatitis.
• SJS/TEN (emergency): keratinocyte apoptosis causes epidermal detachment with fluid loss, sepsis, and late ocular scarring or symblepharon; DRESS (emergency) presents with eosinophilia, transaminitis, and possible myocarditis.
• Granulomatous organ destruction in tuberculosis and hypersensitivity pneumonitis progressing to fibrosis.

• Mechanism shorthand: ACID — Anaphylactic/atopic (I, IgE), Cytotoxic (II, IgG/IgM against cell-surface antigen), Immune complex (III), Delayed (IV, T cells, no antibody). Type IV is the only antibody-independent type and the only one not transferable by serum.
• Single best next step in anaphylaxis is always IM epinephrine to the anterolateral thigh — never diphenhydramine, never steroids, never "observe." The classic distractor is a stem where hypotension and wheeze are treated with antihistamines first.
• Timing is the discriminator: minutes = Type I; hours after transfusion or drug = Type II; days to weeks with fever, rash, and arthralgia = Type III; 48–72 hours = Type IV.
• Immunofluorescence patterns examiners love: linear IgG along the GBM = anti-GBM (Type II); granular/"lumpy-bumpy" = immune complex (Type III). Low C3/C4 favors immune-complex disease.
• Coombs-positive plus spherocytes distinguishes warm autoimmune hemolysis from hereditary spherocytosis (Coombs negative, positive family history, positive EMA binding/osmotic fragility).
• Falling platelets 5–10 days into heparin with new thrombosis = HIT: calculate the 4Ts, stop heparin, and bridge with a non-heparin anticoagulant. Warfarin first is the trap (venous limb gangrene), and prophylactic platelets are wrong.
• Poison ivy and nickel dermatitis are Type IV, not Type I — patch testing, not skin prick testing, is the confirmatory study; a positive tuberculin skin test is the prototypical delayed reaction.
• Mixed-mechanism traps: the tuberculin reaction and granuloma formation are Type IV; hyperacute transplant rejection is Type II; the Arthus reaction after a booster vaccine is a localized Type III; vancomycin flushing and radiocontrast reactions are direct mast cell degranulation and therefore non-IgE pseudoallergy, yet still require epinephrine when criteria for anaphylaxis are met.