Unit 6: Hypersensitivity; Transplantation; Autoimmunity - Subjective Questions

Hypersensitivity refers to an exaggerated or inappropriate immune response to an antigen that results in tissue damage or disease in a sensitized host. Unlike a normal protective immune response, hypersensitivity reactions cause harm to the host.

Gell and Coombs Classification (Four Types):

• Type I (Immediate/Anaphylactic): IgE-mediated reaction involving mast cells and basophils. Occurs within minutes (e.g., anaphylaxis, allergic asthma).
• Type II (Cytotoxic): IgG or IgM antibodies directed against cell-surface or matrix antigens leading to cell destruction (e.g., hemolytic transfusion reactions).
• Type III (Immune Complex): Antigen-antibody complexes deposit in tissues, activating complement and causing inflammation (e.g., serum sickness).
• Type IV (Delayed-Type/Cell-Mediated): T-cell mediated response occurring 24–72 hours after exposure (e.g., contact dermatitis, tuberculin reaction).

Key distinguishing feature: Types I, II, and III are antibody-mediated, whereas Type IV is cell-mediated.

Type I hypersensitivity is an immediate IgE-mediated reaction that occurs in two phases:

1. Sensitization Phase (First Exposure):

• Allergen enters the body and is processed by antigen-presenting cells (APCs).
• Th2 cells are activated and secrete cytokines (IL-4, IL-13) promoting B-cell class switching to IgE.
• IgE binds to high-affinity FcεRI receptors on mast cells and basophils, sensitizing them.
• No symptoms occur during this phase.

2. Effector Phase (Subsequent Exposure):

• Re-exposure to the allergen cross-links adjacent IgE molecules on mast cell surfaces.
• This triggers degranulation and release of mediators.

Mediators Released:

• Preformed mediators: Histamine, heparin, proteases, chemotactic factors.
• Newly synthesized mediators: Leukotrienes (LTC4, LTD4), prostaglandins (PGD2), platelet-activating factor (PAF).
• Cytokines: IL-4, IL-5, TNF-α.

Clinical Effects:

• Early phase (minutes): Vasodilation, increased vascular permeability, smooth muscle contraction, mucus secretion.
• Late phase (2–8 hours): Infiltration of eosinophils and neutrophils causing prolonged inflammation.

Examples: Anaphylaxis, allergic rhinitis, asthma, food allergies, atopic dermatitis.

Type II hypersensitivity (Cytotoxic hypersensitivity) is mediated by IgG or IgM antibodies directed against antigens present on cell surfaces or extracellular matrix.

Mechanisms of Tissue Damage:

• 1. Complement-Mediated Lysis: Antibody binding activates the classical complement pathway, forming the membrane attack complex (MAC) that lyses the target cell.

• 2. Opsonization and Phagocytosis: Antibody (and complement C3b) coats the target cell, promoting phagocytosis by macrophages via Fc and complement receptors.

• 3. Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): NK cells recognize antibody-coated cells via Fc receptors and destroy them without complement.

• 4. Antibody-Mediated Cellular Dysfunction: Antibodies bind to receptors and alter function without destroying the cell (e.g., stimulating or blocking receptors).

Examples:

• Hemolytic transfusion reactions (ABO incompatibility)
• Hemolytic disease of the newborn (Rh incompatibility)
• Autoimmune hemolytic anemia
• Goodpasture's syndrome (anti-basement membrane antibodies)
• Graves' disease (stimulating antibodies to TSH receptor)
• Myasthenia gravis (blocking antibodies to acetylcholine receptor)

Type III hypersensitivity is caused by the deposition of antigen-antibody (immune) complexes in tissues, leading to complement activation and inflammation.

Pathogenesis:

• Soluble antigen combines with antibody (usually IgG) to form immune complexes.
• Under normal conditions, complexes are cleared by phagocytes.
• When present in excess or in intermediate size, complexes deposit in blood vessel walls, joints, kidneys, and skin.
• Deposited complexes activate complement (C3a, C5a — anaphylatoxins).
• These attract neutrophils, which release lytic enzymes and reactive oxygen species, damaging tissues.
• Platelet aggregation and microthrombi formation further worsen damage.

Common Sites of Deposition:

• Kidneys (glomerulonephritis)
• Joints (arthritis)
• Skin (rashes)
• Blood vessels (vasculitis)

Examples:

• Serum sickness — systemic reaction to foreign serum proteins.
• Arthus reaction — localized reaction at injection site.
• Systemic Lupus Erythematosus (SLE) — DNA/anti-DNA complexes.
• Post-streptococcal glomerulonephritis.
• Farmer's lung (hypersensitivity pneumonitis).

Type IV hypersensitivity (Delayed-Type Hypersensitivity, DTH) is a cell-mediated reaction that takes 24–72 hours to develop. It does not involve antibodies but is mediated by sensitized T lymphocytes.

General Mechanism:

• Sensitization phase: Antigen is processed and presented by APCs; T cells become sensitized.
• Effector phase: On re-exposure, sensitized T cells release cytokines that recruit and activate macrophages, causing tissue damage.

Subtypes:

• 1. Contact Hypersensitivity: Mediated by CD8+ and CD4+ T cells. Caused by haptens (e.g., nickel, poison ivy). Results in contact dermatitis.

• 2. Tuberculin-Type Hypersensitivity: Mediated by CD4+ Th1 cells. Seen in the Mantoux/tuberculin skin test; induration develops at 48–72 hours.

• 3. Granulomatous Hypersensitivity: Most clinically important; results from persistent antigen. Chronic macrophage activation leads to granuloma formation (e.g., tuberculosis, leprosy, sarcoidosis).

Key Cytokines Involved: IL-2, IFN-γ, TNF-β, and chemokines that recruit macrophages.

Examples: Tuberculin reaction, contact dermatitis, graft rejection, granulomatous diseases.

Comparison of Hypersensitivity Types:

Summary:

• Types I, II, III are antibody-mediated and can be transferred by serum.
• Type IV is cell-mediated and transferred by T cells.
• Type I is the fastest, while Type IV is the slowest (delayed).

Anaphylaxis is a severe, immediate Type I hypersensitivity reaction resulting from widespread mast cell and basophil degranulation following allergen exposure in a sensitized individual.

Systemic Anaphylaxis:

• A life-threatening, whole-body reaction.
• Triggered by allergens like insect venom, drugs (penicillin), or foods.
• Massive histamine release causes vasodilation, bronchoconstriction, and increased vascular permeability.
• Symptoms: hypotension (anaphylactic shock), difficulty breathing, laryngeal edema, urticaria.
• Can be fatal within minutes if untreated.

Localized Anaphylaxis (Atopy):

• Reaction confined to a specific tissue or organ.
• Depends on the route of allergen entry.
• Examples: allergic rhinitis (hay fever), asthma, urticaria, food allergies.
• Genetically predisposed individuals are called atopic.

Management:

• Epinephrine (adrenaline) — drug of choice; reverses bronchoconstriction and vasodilation.
• Antihistamines — block histamine effects.
• Corticosteroids — reduce late-phase inflammation.
• Maintain airway and provide oxygen/fluids for shock.

Prevention: Avoidance of allergens, desensitization (immunotherapy), and carrying epinephrine auto-injectors.

Graft rejection is an immune response mounted by the recipient against antigens present on the donor graft that are recognized as foreign.

Immunologic Basis:

• Rejection is primarily due to genetic differences between donor and recipient, especially at the Major Histocompatibility Complex (MHC).
• The recipient's immune system recognizes donor MHC molecules as alloantigens.
• Both T-cell mediated (cellular) and antibody-mediated (humoral) responses contribute to rejection.

Major Antigens Responsible:

• 1. MHC / HLA Antigens (Major Histocompatibility Antigens):

  • Class I (HLA-A, B, C): Present on all nucleated cells; recognized by CD8+ T cells.
  • Class II (HLA-DR, DP, DQ): Present on APCs; recognized by CD4+ T cells.
  • These are the strongest barriers to transplantation.

• 2. Minor Histocompatibility Antigens: Non-MHC antigens causing slower rejection.

• 3. ABO Blood Group Antigens: Important in vascularized grafts; mismatch causes hyperacute rejection.

Mechanisms of Recognition:

• Direct allorecognition: Recipient T cells recognize intact donor MHC on donor APCs.
• Indirect allorecognition: Recipient APCs process donor antigens and present them to recipient T cells.

Outcome: Activation of cytotoxic T cells, macrophages, and antibodies leading to graft destruction.

Tissue typing is the process of identifying and matching the HLA (Human Leukocyte Antigen) types of the donor and recipient to minimize the risk of graft rejection.

Purpose:

• To determine histocompatibility between donor and recipient.
• Better HLA matching improves graft survival.

Methods of Tissue Typing:

• 1. Serological Method (Microcytotoxicity/Complement-Dependent Cytotoxicity):

  • Lymphocytes are incubated with a panel of known anti-HLA antibodies plus complement.
  • Cell death (detected by dye uptake) indicates the presence of that HLA antigen.
  • Used for HLA-A, B, C, and DR typing.

• 2. Mixed Lymphocyte Reaction (MLR):

  • Recipient and donor lymphocytes are cultured together.
  • Proliferation indicates HLA class II (HLA-D) incompatibility.
  • Measures functional T-cell response.

• 3. Molecular / DNA-Based Methods (Modern):

  • PCR-SSP (Sequence-Specific Primers)
  • PCR-SSO (Sequence-Specific Oligonucleotides)
  • DNA Sequencing (SBT) — most accurate.
  • These provide high-resolution HLA typing.

• 4. Crossmatching:

  • Recipient serum is tested against donor lymphocytes to detect preformed antibodies.
  • A positive crossmatch contraindicates transplantation (risk of hyperacute rejection).

Additional Test: ABO blood group matching is also essential.

Cell-mediated graft rejection is the primary mechanism of graft destruction, driven mainly by T lymphocytes recognizing donor alloantigens.

Steps in Cell-Mediated Rejection:

• 1. Antigen Recognition (Sensitization):

  • Recipient T cells recognize donor MHC antigens via direct or indirect allorecognition.
  • Occurs in draining lymph nodes and secondary lymphoid tissue.

• 2. T-cell Activation and Proliferation:

  • CD4+ helper T cells recognize MHC class II and secrete cytokines (IL-2, IFN-γ).
  • CD8+ cytotoxic T cells recognize MHC class I on graft cells.
  • IL-2 drives clonal expansion of T cells.

• 3. Effector Phase:

  • Cytotoxic T lymphocytes (CTLs) directly kill graft cells via perforin/granzyme and Fas–FasL pathways.
  • Th1 cells activate macrophages, causing delayed-type hypersensitivity-like damage.
  • Cytokines recruit inflammatory cells that damage graft vasculature and parenchyma.

Role of T Lymphocytes:

• CD8+ T cells: Direct cytotoxic killing of graft cells.
• CD4+ T cells: Orchestrate the response through cytokine secretion, help CTL and macrophage activation.

Evidence: Nude (T-cell deficient) mice cannot reject grafts, and rejection can be transferred by T cells but not serum, confirming the central role of T lymphocytes.

Graft rejection is classified into three main types based on the timing and underlying mechanism:

1. Hyperacute Rejection:

• Onset: Within minutes to hours.
• Mechanism: Preformed antibodies (against ABO or HLA antigens) in the recipient bind to graft endothelium, activating complement.
• Effect: Thrombosis, vascular occlusion, and ischemic necrosis of the graft.
• Prevention: Proper crossmatching and ABO matching.

2. Acute Rejection:

• Onset: Days to weeks (typically within 3 months).
• Mechanism:
  • Acute cellular rejection: Mediated by CD4+ and CD8+ T cells attacking graft cells.
  • Acute humoral (vascular) rejection: Antibodies damage graft blood vessels.
• Effect: Inflammation, parenchymal cell damage, vasculitis.
• Treatment: Immunosuppressive drugs can often reverse it.

3. Chronic Rejection:

• Onset: Months to years.
• Mechanism: Combination of immune (antibodies, T cells) and non-immune factors leading to gradual fibrosis and vascular changes.
• Effect: Progressive loss of graft function, arteriosclerosis, fibrosis.
• Treatment: Difficult to treat; usually irreversible.

Summary: Hyperacute = antibody-mediated (immediate); Acute = mainly T-cell mediated; Chronic = mixed, slow, and progressive.

Grafts are classified based on the genetic relationship between the donor and recipient:

• 1. Autograft (Autologous graft):

  • Tissue transferred from one site to another in the same individual.
  • No rejection as the tissue is genetically identical.
  • Example: Skin graft from thigh to burnt arm.

• 2. Isograft (Syngeneic graft):

  • Graft between genetically identical individuals (identical twins or inbred animals).
  • No rejection due to identical MHC.
  • Example: Kidney transplant between identical twins.

• 3. Allograft (Homograft):

  • Graft between genetically different members of the same species.
  • Most common clinical transplant; rejection occurs due to MHC differences.
  • Requires immunosuppression.
  • Example: Kidney transplant between unrelated humans.

• 4. Xenograft (Heterograft):

  • Graft between different species.
  • Strong and rapid rejection occurs.
  • Example: Pig heart valve transplanted into a human.

Note: The likelihood of rejection increases from autograft (none) → isograft (none) → allograft (moderate) → xenograft (severe).

Autoimmune disorders are conditions in which the immune system fails to distinguish between self and non-self and mounts an immune response against the body's own tissues (self-antigens), causing damage.

Self-Tolerance:

• Normally, the immune system is tolerant to self-antigens through central and peripheral tolerance mechanisms.
• Central tolerance: Deletion of self-reactive lymphocytes in the thymus (T cells) and bone marrow (B cells).
• Peripheral tolerance: Anergy, regulatory T cells (Tregs), and clonal deletion in the periphery.

Mechanisms of Breakdown of Self-Tolerance:

• 1. Molecular Mimicry: Microbial antigens resemble self-antigens; immune response cross-reacts (e.g., rheumatic fever after streptococcal infection).
• 2. Release of Sequestered Antigens: Hidden antigens (e.g., lens protein, sperm) released by trauma trigger immune response.
• 3. Polyclonal Activation: Nonspecific activation of lymphocytes by superantigens or infections.
• 4. Failure of Regulatory Mechanisms: Defective Tregs or apoptosis.
• 5. Altered Self-Antigens: Drugs or infections modify self-proteins.
• 6. Genetic Factors: Certain HLA types predispose to autoimmunity (e.g., HLA-B27).

Contributing Factors: Genetic predisposition, hormonal influences (more common in females), infections, and environmental triggers.

Classification: Organ-specific and systemic (non-organ specific) autoimmune diseases.

Autoimmune diseases are broadly classified into two categories based on the extent of tissue involvement:

Comparison Table:

Organ-Specific Diseases:

• The immune response is directed against antigens unique to a particular organ.
• Example: In Hashimoto's thyroiditis, antibodies target thyroid antigens (thyroglobulin, thyroid peroxidase).

Systemic Diseases:

• The immune response targets antigens present in many tissues, causing widespread damage.
• Example: In SLE, antibodies against DNA and nuclear antigens form immune complexes that deposit in multiple organs.

Note: Some diseases fall in an intermediate category (e.g., Goodpasture's syndrome).

Systemic Lupus Erythematosus (SLE) is a chronic systemic autoimmune disease characterized by the production of autoantibodies against nuclear and cytoplasmic antigens, resulting in multi-organ damage. It predominantly affects young women.

Pathogenesis:

• Loss of self-tolerance leads to production of autoantibodies, especially anti-nuclear antibodies (ANA) and anti-dsDNA antibodies.
• Autoantibodies combine with self-antigens to form immune complexes (Type III hypersensitivity).
• These complexes deposit in kidneys, skin, joints, and blood vessels, activating complement and causing inflammation.
• Type II hypersensitivity also contributes (antibodies against blood cells).

Clinical Features:

• Butterfly (malar) rash across cheeks and nose.
• Arthritis and joint pain.
• Glomerulonephritis (lupus nephritis) — major cause of morbidity.
• Photosensitivity, oral ulcers.
• Hematologic: anemia, leukopenia, thrombocytopenia.
• Serositis (pleuritis, pericarditis), neurological and cardiac involvement.

Diagnosis:

• Anti-nuclear antibody (ANA) test — screening (highly sensitive).
• Anti-dsDNA and anti-Sm antibodies — highly specific.
• Reduced complement levels (C3, C4).
• LE cell phenomenon (historical).
• Clinical criteria (ACR/SLICC classification).

Treatment: Corticosteroids, immunosuppressants, and antimalarials (hydroxychloroquine).

Both Myasthenia Gravis and Graves' disease are organ-specific autoimmune diseases involving antibodies against cell-surface receptors (Type II hypersensitivity, antibody-mediated receptor dysfunction).

1. Myasthenia Gravis:

• Target: Autoantibodies against acetylcholine receptors (AChR) at the neuromuscular junction.
• Mechanism: Antibodies block and destroy AChR, preventing nerve impulse transmission to muscles (blocking antibodies).
• Clinical Features: Progressive muscle weakness and fatigue, especially of ocular, facial, and limb muscles; drooping eyelids (ptosis), double vision.
• Diagnosis: Anti-AChR antibody test, edrophonium (Tensilon) test, EMG.
• Treatment: Anticholinesterase drugs, immunosuppression, thymectomy.

2. Graves' Disease:

• Target: Autoantibodies against the TSH receptor on thyroid cells.
• Mechanism: Antibodies stimulate the TSH receptor (stimulating antibodies), mimicking TSH and causing excessive thyroid hormone production (hyperthyroidism).
• Clinical Features: Weight loss, heat intolerance, tachycardia, goiter, and exophthalmos (bulging eyes).
• Diagnosis: Elevated T3/T4, low TSH, presence of thyroid-stimulating immunoglobulins (TSI).
• Treatment: Antithyroid drugs, radioactive iodine, surgery.

Key Difference: In Myasthenia Gravis, antibodies block receptors (loss of function), whereas in Graves' disease, antibodies stimulate receptors (gain of function).

The Arthus reaction is a localized Type III hypersensitivity reaction occurring at the site of antigen entry in a previously sensitized individual with high levels of circulating antibody.

Mechanism:

• Local injection of antigen into a sensitized host with high IgG levels.
• Antigen combines with antibody to form immune complexes locally.
• Complexes activate complement (C3a, C5a) and attract neutrophils.
• Neutrophils release lytic enzymes, causing localized vasculitis, edema, hemorrhage, and necrosis.
• Develops within 4–8 hours.

Comparison with Serum Sickness:

Common Basis: Both are Type III immune complex-mediated reactions involving complement activation and neutrophil-mediated damage.

Hemolytic Disease of the Newborn (HDN), also called erythroblastosis fetalis, is a Type II hypersensitivity reaction caused by maternal antibodies against fetal red blood cell antigens, most commonly the Rh (D) antigen.

Mechanism:

• Occurs when an Rh-negative mother carries an Rh-positive fetus.
• During the first pregnancy or delivery, fetal Rh+ RBCs enter maternal circulation.
• The mother becomes sensitized and produces anti-Rh (IgG) antibodies.
• In a subsequent Rh+ pregnancy, maternal IgG antibodies cross the placenta.
• These antibodies bind to fetal RBCs, causing complement-mediated lysis and destruction.

Clinical Features:

• Fetal anemia, jaundice, and hyperbilirubinemia.
• Severe cases: hydrops fetalis (edema, heart failure), kernicterus (brain damage from bilirubin).

Prevention:

• Administration of anti-Rh (D) immunoglobulin (RhoGAM) to Rh-negative mothers.
• Given at 28 weeks of gestation and within 72 hours after delivery of an Rh+ baby.
• The antibody binds and clears fetal Rh+ cells before the mother can be sensitized, preventing antibody formation.

Treatment (if it occurs): Intrauterine transfusion, exchange transfusion, and phototherapy for jaundice.

Immunosuppressive therapy is used after transplantation to suppress the recipient's immune response and prevent or control graft rejection, thereby prolonging graft survival.

Goals:

• Prevent rejection while maintaining enough immunity to fight infections.
• Achieve a balance between rejection and over-immunosuppression.

Major Classes of Immunosuppressive Agents:

• 1. Corticosteroids (e.g., Prednisone):

  • Broadly anti-inflammatory; inhibit cytokine gene transcription and reduce leukocyte activity.

• 2. Calcineurin Inhibitors (e.g., Cyclosporine, Tacrolimus):

  • Block IL-2 production and T-cell activation by inhibiting calcineurin.
  • Mainstay of transplant immunosuppression.

• 3. Antiproliferative/Cytotoxic Agents (e.g., Azathioprine, Mycophenolate mofetil):

  • Inhibit DNA synthesis, blocking lymphocyte proliferation.

• 4. mTOR Inhibitors (e.g., Sirolimus/Rapamycin):

  • Block IL-2 signal transduction and T-cell proliferation.

• 5. Biological Agents / Antibodies:

  • Anti-thymocyte globulin (ATG) — depletes T cells.
  • Anti-CD3 (OKT3), Anti-CD25 (Basiliximab) — target specific immune molecules.

Side Effects:

• Increased risk of infections and malignancies.
• Drug-specific toxicities (nephrotoxicity, hypertension, etc.).

Note: Combination therapy is used to reduce individual drug toxicity while enhancing effectiveness.

Rheumatoid Arthritis (RA) is a chronic systemic autoimmune disease primarily affecting the synovial joints, leading to inflammation, cartilage destruction, and joint deformity. It is more common in women.

Immunopathogenesis:

• Loss of self-tolerance leads to activation of autoreactive T cells (Th1 and Th17) and B cells in the synovium.
• Production of Rheumatoid Factor (RF) — an autoantibody (usually IgM) directed against the Fc region of the patient's own IgG.
• RF and IgG form immune complexes (Type III hypersensitivity) that deposit in joints and activate complement.
• Activated macrophages and T cells release proinflammatory cytokines (TNF-α, IL-1, IL-6), driving chronic inflammation.
• Formation of pannus (inflammatory granulation tissue) that erodes cartilage and bone.

Clinical Features:

• Symmetrical joint pain, swelling, and morning stiffness.
• Involvement of small joints of hands and feet.
• Joint deformities in advanced disease.
• Systemic features: fatigue, rheumatoid nodules, vasculitis.

Diagnostic Markers:

• Rheumatoid Factor (RF) — positive in most patients.
• Anti-CCP (anti-cyclic citrullinated peptide) antibodies — highly specific.
• Elevated ESR and CRP (markers of inflammation).
• Radiographic evidence of joint erosion.

Treatment: NSAIDs, DMARDs (methotrexate), and biologics (anti-TNF agents).