Hypersensitivity Types I–III: Pathophysiology, Diseases, and Treatments

Type I Hypersensitivity (IgE-mediated)

  • Overview

    • Immediate-type allergic reaction driven by IgE antibodies and mast cell degranulation.
    • Mediators released: histamine, leukotrienes, prostaglandins, and other inflammatory mediators.
    • Regardless of the tissue site (respiratory, skin, etc.), signs/symptoms arise from mediator effects on vasculature, smooth muscle, and mucous membranes.

  • Pathophysiology (key sequence)

    • Antigen exposure leads to IgE class switching and production by B cells.
    • IgE binds to high-affinity FcεRI receptors on mast cells and basophils.
    • Upon re-exposure, cross-linking of bound IgE triggers degranulation and mediator release.
    • Resulting clinical manifestations depend on tissue and mediator profile.
    • Fundamental equation representing the cascade:
      extAntigenexposure→IgE production→FcεRI cross-linking→mast cell degranulation→mediator releaseext{Antigen exposure} \rightarrow \text{IgE production} \rightarrow \text{Fc}\varepsilon\text{RI cross-linking} \rightarrow \text{mast cell degranulation} \rightarrow \text{mediator release}

  • Clinical examples (typical presentations in primary care)

    • Anaphylaxis (emergency).
    • Bee stings, severe food allergies (e.g., peanut).
    • Allergic rhinitis and related allergic diseases.
    • These conditions arise from mediator-induced effects on vascular permeability, bronchoconstriction, and mucus production.

  • Diagnostic approaches (non-diagnostic course context)

    • Skin testing: introduction of suspected allergens into the skin via needle prick or intradermal injection.
    • Serum IgE levels: quantify circulating allergen-specific IgE.
    • RAST / ImmunoCAP-like testing (RAS testing mentioned): detects specific IgE antibodies to particular antigens in blood.
    • Important note on scope: this is a pathophysiology-focused course; not a diagnostic credentialing course.

  • Treatments and management (pathophysiology-aligned)

    • Avoidance of known allergens when possible.
    • Antihistamines to block histamine effects on H1 receptors and reduce symptoms.
    • Corticosteroids to reduce inflammation and immune response.
    • Desensitization immunotherapy (allergy shots): gradual exposure to increase tolerance.
    • Acute management for systemic reactions: Epinephrine (adrenaline) intramuscular injection (EpiPen) as life-saving treatment for anaphylaxis.
    • Other formulations of corticosteroids (injections, pills, sprays) depending on site (e.g., fluticasone nasal spray for allergic rhinitis, hydrocortisone cream for skin reactions).

  • Educational/clinical reasoning points

    • Patient education emphasizes recognizing triggers and early treatment to prevent progression to anaphylaxis.
    • Even though pathophysiology is foundational, diagnosing allergic disease should be approached with clinical reasoning and not purely by test results in this course.
    • The same mediator-driven logic underpins management decisions across hypersensitivity types.

Type II Hypersensitivity (Cytotoxic Reaction)

  • Overview

    • Involves destruction of host cells via antibodies directed at cell-surface antigens.
    • Primarily mediated by IgG or IgM antibodies targeting antigens on the surface of self-cells.
    • Outcomes include cell lysis, altered cell function, and inflammatory responses.

  • Pathophysiology (core mechanisms)

    • Antibodies bind to surface antigens on host cells.
    • Complement activation via the classical pathway leads to membrane attack complex (MAC) formation and cell lysis.
    • Antibody-dependent cellular cytotoxicity (ADCC): NK cells recognize Fc region of bound antibodies and release cytotoxic granules (e.g., perforin, granzymes) to destroy the target cell.
    • Phagocytosis: macrophages recognize Fc regions (Fc receptors) and engulf antibody-coated cells (opsonization enhances engulfment).
    • Core conceptual chain:
      extIgG/IgM+extcell−surfaceantigen→complement activation (classical pathway)→MAC→cell lysisext{IgG/IgM} + ext{cell-surface antigen} \rightarrow \text{complement activation (classical pathway)} \rightarrow \text{MAC} \rightarrow \text{cell lysis}
    • ADCC pathway:
      extNKcell→extFcreceptorbindingtoantibody−coatedtarget→cytotoxic granule release→target cell deathext{NK cell} \xrightarrow{} ext{Fc receptor binding to antibody-coated target} \rightarrow \text{cytotoxic granule release} \rightarrow \text{target cell death}

  • Typical diseases (examples)

    • Hemolytic anemia (autoimmune hemolytic anemia, AIHA): antibodies target RBCs leading to RBC destruction.
    • Graves’ disease (autoimmune thyroid disease with autoimmune stimulation rather than destruction of thyroid tissue).
    • Myasthenia gravis (autoantibodies block acetylcholine receptors at the neuromuscular junction).
    • Hemolytic disease of the newborn (maternal antibodies target fetal RBCs).
    • Other autoimmune conditions such as Goodpasture's disease (autoantibodies against basement membranes).

  • Signs and symptoms (domain-specific examples)

    • Hemolytic anemia: fatigue, pallor, jaundice, dark urine, tachycardia, dyspnea, splenomegaly.
    • Consider cross-system manifestations depending on the affected tissue (e.g., neuromuscular symptoms in MG, hyperthyroid signs in Graves’).

  • Diagnostics (grounded in cellular mechanisms)

    • Direct Coombs test: detects antibodies bound to RBCs in vivo.
    • Indirect Coombs test: detects free circulating antibodies in serum (often used in prenatal testing and transfusion medicine).
    • Contextual interpretation: use alongside clinical presentation to identify cytotoxic mechanisms in disease.

  • Treatments and management (pathophysiology-directed)

    • Corticosteroids to suppress immune response and inflammation.
    • Immunosuppressants for steroid-resistant or relapsing disease (stronger suppression).
    • Intravenous immunoglobulin (IVIG): competes with autoantibodies, reducing Fc receptor–mediated clearance and immune activity.
    • Blood transfusions with careful cross-matching due to autoantibodies; caution to avoid exacerbating hemolysis.
    • Splenectomy for refractory cases since spleen is a major site of RBC destruction.
    • For Graves’ disease specifically (while still Type II context), thyroid-directed therapies are used (see Graves’ disease section).

  • Graves’ disease (Type II hypersensitivity subset)

    • Autoantibodies (TSI: thyroid-stimulating immunoglobulins) mimic TSH by binding to TSH receptors on thyroid cells.
    • Result: excessive stimulation of thyroid hormone production; TH overproduction; thyroid gland not destroyed.
    • Pathophysiology players: T cells (Th2) promote B cell activation and autoantibody production; cytokines (e.g., IL-4, IL-10) sustain the autoimmune response.
    • Ophthalmopathy (exophthalmos) due to orbital fibroblast and T cell involvement; infiltration by immune cells contributes to eye symptoms.
    • Presentation: hyperthyroidism signs (weight loss, heat intolerance, sweating, tachycardia, anxiety, tremor, goiter) plus exophthalmos and menstrual irregularities.
    • Diagnostics and treatment considerations: standard thyroid tests; treatment includes antithyroid drugs (e.g., Methimazole, PTU), beta-blockers for symptom control, radioactive iodine therapy, or surgery; corticosteroids may be used for orbital involvement.

  • Diagnostic caveats and clinical reasoning

    • Do not over-rely on a single test or symptom to label a diagnosis; use clinical reasoning and consider cross-system involvement.
    • Recognize overlap with other hypersensitivity pathways (e.g., lupus presenting with hematologic abnormalities may involve both Type II and III mechanisms).

Type III Hypersensitivity (Immune Complex-mediated)

  • Overview

    • Formation of antigen-antibody (IgG or IgM) immune complexes that circulate and deposit in tissues rather than binding on cell surfaces.
    • Deposited complexes activate complement and recruit neutrophils, leading to inflammation and tissue damage.

  • Pathophysiology (mechanistic outline)

    • Antigen + antibody complexes form in circulation.
    • Complexes deposit in tissues (kidneys, joints, small vessels, skin, lungs, etc.).
    • Complement activation and neutrophil recruitment drive inflammatory injury.
    • Typical pattern: tissue injury from immune-complex deposition rather than direct cell-surface targeting.
    • Representative sequence:
      extAntigen+extIgG/IgM→immune complex deposition in tissues→complement activation→neutrophil recruitment and inflammation→tissue damageext{Antigen} + ext{IgG/IgM} \rightarrow \text{immune complex deposition in tissues} \rightarrow \text{complement activation} \rightarrow \text{neutrophil recruitment and inflammation} \rightarrow \text{tissue damage}

  • Localizations and presenting signs by site

    • Kidneys: glomerulonephritis with proteinuria, hematuria, potential renal failure (e.g., lupus nephritis, post-streptococcal GN).
    • Joints: arthritis and joint pain (e.g., systemic lupus erythematosus involvement).
    • Skin: rash, purpura, serum sickness-like reactions.
    • Blood vessels: vasculitis.
    • Lungs: alveolitis, hemorrhage, pneumonitis when complexes deposit in pulmonary tissue.

  • Classic disease examples

    • Systemic lupus erythematosus (SLE): widespread involvement with immune complexes affecting multiple organ systems.
    • Post-streptococcal glomerulonephritis (PSGN).
    • Serum sickness and Arthus reactions (hypersensitivity reactions linked to immune complex deposition).

  • Lupus: Type II and Type III overlap

    • Lupus is primarily a Type III hypersensitivity disease due to immune complex deposition, but some hematologic features involve Type II mechanisms (autoantibodies against blood cell antigens causing cytopenias).
    • A representative schematic shows systemic involvement across organ systems with corresponding signs/symptoms and the dual presence of Type II and Type III features.
    • Diagnostic and therapeutic approaches reflect the mixed nature, including NSAIDs or corticosteroids for inflammation, immunosuppressants, and disease-modifying strategies.

  • Diagnostic considerations and clinical reasoning

    • The pattern of deposition and organ involvement informs the hypersensitivity type attribution.
    • Do not assume a single mechanism; lupus can exhibit features of both Type II and Type III processes depending on the organ system.
    • The large, integrative table discussed in the lecture illustrates how lupus can affect hematologic, renal, cutaneous, and musculoskeletal systems with corresponding signs.

  • Therapy and management approach (general for Type III-associated diseases)

    • Anti-inflammatory strategies (NSAIDs, corticosteroids).
    • Immunosuppressive agents for ongoing autoimmune activity.
    • Supportive care targeted to organ involvement (e.g., nephrology for lupus nephritis).

Graves’ Disease and Hematologic/Endocrine Connections (Type II focus)

  • Graves’ disease – focused pathophysiology (Type II) with endocrine implications

    • Autoantibodies (TSI) mimic TSH and bind TSH receptors on thyroid cells.
    • Result: excessive thyroid hormone production with hyperthyroidism; thyroid tissue not destroyed.
    • Immune cell involvement: Th2 cells promote B cell activation and autoantibody production; cytokines (IL-4, IL-10) sustain response.
    • Extra-thyroidal manifestations may include exophthalmos due to orbital tissue involvement.

  • Clinical features and management

    • Hyperthyroid symptoms: weight loss, heat intolerance, sweating, tachycardia, anxiety, tremor, goiter.
    • Eye signs: exophthalmos due to orbital fibroblast/T-cell activity.
    • Diagnostic approach: thyroid function tests; autoantibody testing (TSI).
    • Treatments include antithyroid medications (e.g., Methimazole, PTU), beta-blockers for symptom control, radioactive iodine, or thyroidectomy, with corticosteroids as needed for orbital involvement.

  • Hematologic considerations in Type II context

    • Autoantibody-mediated cytopenias (e.g., autoimmune hemolytic anemia) showcase Type II mechanisms in the blood/hematologic system.
    • Management often starts with steroids; escalation to immunosuppressants or IVIG if refractory.

Hemolytic Anemia (Type II) – Key Features and Reasoning

  • Mechanism

    • Autoantibodies target RBC surface antigens (IgM or IgG), leading to RBC destruction via complement or phagocytosis.
    • IgM can strongly activate the classical complement pathway; IgG often engages Fc receptors on phagocytes.
    • Result: hemolysis, anemia, and related symptoms.

  • Clinical presentation (typical signs to recognize)

    • Fatigue, weakness, pallor, jaundice, dark urine, tachycardia, dyspnea.
    • Splenomegaly from increased RBC destruction in spleen.

  • Diagnostic tests and interpretation

    • Direct Coombs test: detects antibodies bound to RBCs in vivo.
    • Indirect Coombs test: detects free antibodies in serum; used in prenatal testing and transfusion compatibility.

  • Management principles

    • Corticosteroids to suppress immune activity; escalate to stronger immunosuppressants if needed.
    • IVIG in acute/severe cases to modulate autoantibody effects.
    • Blood transfusions with careful cross-matching due to circulating autoantibodies.
    • Splenectomy for refractory cases since splenic destruction is a major pathway.

Diagnostic Reasoning and Education Across Hypersensitivity Types

  • Integrated clinical reasoning

    • Do not jump to a single diagnosis based on symptoms or a single clue (e.g., Wilbur case exercise); use a systematic clinical reasoning process to evaluate competing hypotheses.
    • Pathophysiology knowledge helps explain signs/symptoms and guides rational treatment rather than solely chasing a label.
    • Fellows should connect concepts across organ systems (hematologic, endocrine, renal, dermatologic) to understand how immune dysregulation manifests in multiple contexts.

  • Links to pharmacology and therapeutics

    • Across Type I–III, inflammation underlies pathology; corticosteroids are frequently used as a first-line anti-inflammatory/immunosuppressive agent.
    • If steroids fail or relapse occurs, immunosuppressive agents or IVIG may be implemented.
    • For Type I, epinephrine is life-saving in anaphylaxis; for autoimmune cytotoxic processes (Type II/III), disease-modifying approaches are used to reduce autoantibody production and tissue injury.

  • Practical implications and patient care considerations

    • Educate patients on allergen avoidance, symptom recognition, and emergency action plans for anaphylaxis.
    • Exercise caution with transfusions in hemolytic anemias due to autoantibodies; ensure cross-matching and compatibility.
    • Consider multidisciplinary care for complex diseases (e.g., lupus requiring rheumatology, nephrology; Graves’ with endocrinology and ophthalmology).

Connections to Foundational Principles and Real-World Relevance

  • Foundational links

    • Type I hinges on IgE class switching and mast cell biology, illustrating how signaling and mediator release translate to clinical symptoms.
    • Type II highlights how antibodies can exert protective and destructive roles, including direct cell targeting and interference with receptor signaling.
    • Type III demonstrates why circulating immune complexes cause systemic inflammation and multi-organ disease).
    • Lupus exemplifies how autoimmune processes can span multiple hypersensitivity pathways and organ systems, reinforcing the overlap in real-world disease.

  • Relevance to pharmacology, diagnostics, and patient safety

    • Knowledge of these mechanisms informs rational drug choices (e.g., steroids, immunosuppressants, IVIG) and helps anticipate side effects.
    • Diagnostic tests (skin testing, IgE assays, Coombs tests) are tools to support, not replace, clinical reasoning.
    • Ethical and practical implications include accurate diagnosis, appropriate referrals, and avoiding overtreatment or mislabeling of diseases based on partial data.

  • Final take-home messages

    • Hypersensitivity reactions are defined by the immune mechanism and tissue targets rather than by a single organ system.
    • Overlaps between Type II and Type III occur in diseases like lupus and Graves’ disease, illustrating the complexity of immune-mediated pathology.
    • A solid understanding of cellular and molecular mechanisms makes it easier to anticipate clinical manifestations, select appropriate tests, and craft effective treatment plans.