Hypersensitivity

Overview of Hypersensitivity and Immune Function

• Definition of Hypersensitivity:

  • A normal immune response that is inappropriately triggered, excessive, and produces undesirable or harmful effects on the host body.

  • Represents an immune reaction directed against an otherwise harmless molecule encountered in the environment.

  • Upon initial exposure, the host undergoes sensitization; upon subsequent re-exposure to the same antigen, an acute inflammatory reaction is triggered.

• Review of Core Immune System Components:

  • Antigens:

  • Any substance (typically a protein or carbohydrate) recognized by the immune system as foreign.

  • Capable of binding specifically to immune receptors and triggering an adaptive immune response.

  • Lymphocytes:

  • B-Lymphocytes (B-Cells):

    • Differentiate into plasma cells to produce specific immunoglobulins (antibodies).

    • Antibodies bind specifically to target antigens to neutralize or facilitate their destruction.

  • T-Lymphocytes (T-Cells):

    • Express T-cell receptors (TCRs) that recognize specific peptide antigens bound to major histocompatibility complex (MHC) molecules.

    • Cytotoxic T-Cells ($CD8^+$): Directly toxic to target cells displaying specific surface antigens, causing cell destruction.

    • Helper T-Cells ($CD4^+$): Assist and activate other immune components, including B-lymphocytes and macrophages.

• Fundamental Hypersensitivity Triggers:

  • Antigen-Antibody Reactions (Antibody-Mediated): Types I, II, and III hypersensitivity reactions.

  • Antigen-Lymphocyte Reactions (T-Cell Mediated): Type IV hypersensitivity reaction.

Type I Hypersensitivity (IgE-Mediated Reaction)

• General Characteristics and Onset:

  • Immediate hypersensitivity reaction occurring within $15\text{--}20\,\text{minutes}$ of re-exposure to an antigen.

  • Requires prior exposure to the antigen to induce initial sensitization.

• Common Antigens and Environmental Triggers:

  • Environmental Allergens: Animal dander, bee venom.

  • Food Allergens: Nuts, seafood, eggs.

  • Medications: Penicillin, radiocontrast dyes.

• Etiology and Genetics:

  • Strong genetic link (atopy):

  • Children with 1 allergic parent have a $2\text{--}3\times$ higher risk of developing Type I allergies.

  • Children with 2 allergic parents have a $5\text{--}10\times$ higher risk.

  • Identical twins demonstrate a $50\%$ concordance rate.

  • Primary Cellular Components: B-lymphocytes, Plasma cells, IgE antibodies (immunoglobulins), and Mast cells.

• Pathogenesis Sequence:

  1. Initial exposure to an environmental antigen.

  2. The antigen is processed and presented by antigen-presenting cells (APCs) to helper T-cells ($CD4^+$).

  3. Helper T-cells stimulate naive B-lymphocytes.

  4. B-lymphocytes become activated and differentiate into plasma cells.

  5. Plasma cells produce and secrete antigen-specific IgE antibodies.

  6. Secreted IgE antibodies bind via their Fc region to high-affinity Fc receptors on the surface of tissue mast cells (and circulating basophils), creating "sensitized" mast cells.

  7. On subsequent re-exposure, the allergen binds to and cross-links adjacent IgE antibodies on the sensitized mast cell membrane.

  8. IgE cross-linking triggers mast cell degranulation, causing the cell to dump pre-formed intracellular granules containing histamine and other inflammatory mediators directly into surrounding tissues.

• Inflammatory Mediators and Clinical Manifestations:

  • Potent Vasodilation: Manifests as nasal congestion (stuffy nose), systemic hypotension (lowered blood pressure), and skin wheals.

  • Increased Vascular Permeability: Causes fluid extravasation leading to tissue edema and rhinorrhea (runny nose).

  • Bronchial Smooth Muscle Constriction: Causes airway narrowing, breathing difficulties, and wheezing.

  • Irritant Receptor Stimulation: Triggers intense cutaneous itching (pruritus).

• Localized vs. Systemic Type I Reactions:

  • Localized (Atopic) Reactions:

  • Allergic Rhinitis: Seasonal hay fever caused by airborne allergens.

  • Asthma: Lower airway hyperresponsiveness and bronchospasm.

  • Urticaria (Hives): Cutaneous wheal-and-flare reactions.

  • Common Local Triggers: Pollen, dust mites, mold spores, animal dander.

  • Systemic Reactions (Anaphylaxis):

  • Characterized by massive systemic release of chemical mediators.

  • Leads to profound bronchial constriction, severe airway obstruction, and generalized vascular collapse (anaphylactic shock).

  • Common Systemic Triggers: Systemic medications (e.g., penicillin), bee venom, specific foods (e.g., peanuts, shellfish).

• Organ System Manifestations:

  • Intravascular System: Anaphylactic shock due to widespread vasodilation and plasma loss.

  • Skin: Urticaria (hives), angioedema, atopic dermatitis, wheal-and-flare reactions.

  • Respiratory System: Rhinitis, bronchospasm, asthma.

  • Gastrointestinal System: Nausea, vomiting, abdominal cramping pain, diarrhea.

Type II Hypersensitivity (Cytotoxic / Antibody-Mediated Reactions)

• Mechanism of Action:

  • Cell-surface markers (antigens) on host cells or foreign cells stimulate the production of specific antibodies.

  • Antibodies (IgG or IgM) recognize, bind, and attach directly to cell-surface antigens.

  • Antigen-antibody binding causes direct cell destruction via complement-mediated cell lysis, phagocytosis, or Antibody-Dependent Cellular Cytotoxicity (ADCC) involving cytotoxic T-cells, natural killer (NK) cells, macrophages, and neutrophils.

• Complement Cascade Pathway Detail:

  • Binding of IgG or IgM antibodies to cellular antigens activates the classical complement pathway via the C1 complex.

  • Generates C2a and C4b fragments, assembling C3 convertase.

  • Mediates C3 hydrolysis into C3a (anaphylatoxin) and C3b (opsonin) fragments.

  • C3b cleaves C5 into C5a and C5b.

  • C5b, C6, C7, C8, and multiple C9 molecules aggregate to form the cylindrical Membrane Attack Complex (MAC).

  • The MAC inserts into the target cell membrane, creating a pore that causes intracellular influx, cell swelling, and bursting (lysis).

• Target Antigens and Representative Clinical Conditions:

  • Antigens Involved: Red blood cell membrane markers, specific tissue receptors, or autologous cell markers.

  • Autoimmune Hemolytic Anemia: Autoantibody-mediated destruction of host erythrocytes.

  • Erythroblastosis Fetalis (Hemolytic Disease of the Newborn):

  1. An Rh-negative mother conceives an Rh-positive fetus.

  2. Rh-positive fetal red blood cells enter the mother's bloodstream during gestation or delivery.

  3. The mother becomes sensitized and produces anti-Rh IgG antibodies (\\diamond\).

  4. In a subsequent Rh-positive pregnancy, maternal anti-Rh IgG antibodies cross the placenta into fetal circulation.

  5. Maternal IgG antibodies attack fetal Rh-positive red blood cells, causing severe hemolytic anemia and erythroblastosis fetalis.

  • Myasthenia Gravis:

  • Autoantibodies (anti-AChR) target and bind to nicotinic acetylcholine (ACh) receptors at the postsynaptic neuromuscular junction.

  • Blocks acetylcholine binding and triggers receptor degradation, leading to muscle weakness.

• Mismatched Blood Transfusion Reactions:

  • Transfusion of Type A donor red blood cells into a Type B recipient who possesses circulating anti-A isohemagglutinins.

  • Anti-A antibodies in recipient plasma bind donor Type A red blood cells.

  • Activates the complement cascade, causing massive intravascular hemolysis and free hemoglobin release.

  • Blood Transfusion Reaction Categories and Manifestations:

• Hemolytic Reaction: Flank pain, fever, chills, hypotension, hemoglobinuria.

• Febrile Non-Hemolytic Reaction: Chills, fever, headache.

• Allergic Reaction: Urticaria, pruritus (itching), skin flushing.

• Septic Reaction: High fever, severe chills, hypotension, septic shock.

Type III Hypersensitivity (Immune Complex-Mediated Reactions)

• Mechanism of Action:

  • Antigen-antibody complexes (formed by soluble antigens binding with IgG or IgM antibodies) form in circulation.

  • Instead of being cleared by the reticuloendothelial system, these immune complexes deposit into vascular basement membranes and surrounding tissue matrices.

  • Local immune complex deposition initiates an intense inflammatory cascade resulting in severe tissue damage.

• Inflammatory Cascade and Pathophysiology:

  1. Circulating soluble immune complexes deposit in the walls of blood vessels or capillary beds.

  2. Deposited complexes activate complement, generating anaphylatoxins C3a, C4a, and C5a.

  3. Complement activation attracts inflammatory white blood cells, primarily neutrophils, and induces mast cell degranulation.

  4. Recruited neutrophils attempt to phagocytose the embedded immune complexes but are unable to internalize them ("frustrated phagocytosis").

  5. Neutrophils release lysosomal enzymes, reactive oxygen species, and proinflammatory mediators, damaging basement membranes, endothelial cells, and adjacent parenchymal tissue.

• Triggers and Common Antigens:

  • Foreign Antigens: Bacterial or viral proteins, therapeutic drugs (e.g., penicillin G benzathine), heterologous antivenoms, vaccines.

  • Self-Antigens (Autoantigens): Endogenous nuclear DNA, structural cell receptors.

• Clinical Manifestations and Disease Examples:

  • Clinical features depend directly on the site of immune complex deposition.

  • Systemic Lupus Erythematosus (SLE):

  • Driven by anti-DNA antibodies, anti-Sm antibodies, antinuclear antibodies (ANAs), and antiphospholipid antibodies.

  • Causes classic facial butterfly rash, discoid rash, lupus nephritis (characterized by wire-loop capillary lesions in glomeruli), and Libman-Sacks endocarditis.

  • Arthus Reaction:

  • Localized Type III hypersensitivity experimentally or clinically induced in the skin.

  • Occurs 1–2 hours after injection of an antigen into an individual with high levels of circulating IgG antibodies, causing local immune-complex vasculitis, fluid and protein extravasation, and tissue necrosis.

  • Other Disease Examples: Post-streptococcal glomerulonephritis, Rheumatoid arthritis.

Type IV Hypersensitivity (Delayed / T-Cell-Mediated Reactions)

• Mechanism of Action:

  • Distinct from Types I–III because it involves NO ANTIBODIES.

  • Mediated entirely by cellular immune mechanisms involving T-lymphocytes ($CD4^+$ helper T-cells and $CD8^+$ cytotoxic T-cells).

  • Demonstrates a delayed onset, typically taking $24\text{--}72\,\text{hours}$ to manifest fully after antigen exposure.

• Two-Phase Pathogenesis:

  • Sensitization Phase:

  1. Small, incomplete antigens known as haptens penetrate the cutaneous barrier.

  2. Haptens bind covalently to endogenous serum/tissue proteins to form complete immunogenic antigens.

  3. Cutaneous Antigen-Presenting Cells (APCs, such as Langerhans cells/dendritic cells) recognize, internalize, and process the hapten-protein complex.

  4. APCs present haptenized peptides to naive $T_H1$ helper T-cells.

  • Effector Phase:

  1. Subsequent re-exposure activates memory $T_H1$ helper T-cells.

  2. Activated $T_H1$ cells secrete proinflammatory cytokines, including Interleukin-1 (IL-1), Interleukin-2 (IL-2), Interferon-gamma (INF-$\gamma$), Tumor Necrosis Factor-alpha (TNF-$\alpha$), and Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF).

  3. Released cytokines recruit and activate macrophages and $CD8^+$ cytotoxic T-cells.

  4. Activated macrophages and cytotoxic T-cells release lytic enzymes and granzymes, producing tissue inflammation, lesion formation, and cellular destruction.

• Common Antigens and Triggers:

  • Plant urushiol oils (poison ivy, poison oak, poison sumac).

  • Topical cosmetics, clothing dyes, industrial adhesives.

  • Heavy metals (nickel alloys in jeans buttons/rivets, causing umbilical rash).

  • Tuberculin antigen (purified protein derivative / PPD test).

  • Dietary gluten (celiac disease).

  • Transplanted tissue or organ allografts (graft rejection).

• Clinical Manifestations:

  • Contact Dermatitis: Pruritic skin rash characterized by erythema, papules, and fluid-filled vesicles or blisters.

  • Tuberculin Test Reaction: Cutaneous induration (hardness) and redness at the intradermal injection site.

  • Systemic Inflammatory Response: Fever, generalized body aches, malaise.

  • Neurological Deficits: Occurs when self-reactive T-cells attack and demyelinate the myelin sheath (e.g., Multiple Sclerosis).

  • Severe Cutaneous Adverse Reactions: Drug-induced cytotoxic cell granzyme release causing Stevens-Johnson Syndrome (SJS, involving $<10\%$ epidermal detachment) or Toxic Epidermal Necrolysis (TEN, involving $>30\%$ epidermal detachment).

Comparative Summary of Hypersensitivity Types

Pharmacological Management of Hypersensitivity & Allergic Reactions

• General Therapeutic Strategies:

  • Immunosuppressants: Suppress cell-mediated immune responses and lymphokine production.

  • Anti-Inflammatory Medications: Decrease overall inflammatory cascade activity.

  • Corticosteroids: Suppress cytokine transcription and leukocyte accumulation (e.g., topical Hydrocortisone cream $1\%$ for minor skin irritations and anti-itch relief).

• Antihistamines: Release Blockers vs. Receptor Antagonists:

  • Mast Cell Stabilizers / Release Blockers:

  • Cromolyn sodium: Prevents mast cell degranulation; useful in asthma and COPD management.

  • Leukotriene modifiers: Block leukotriene synthesis or receptors to reduce bronchoconstriction.

  • Histamine Receptor Antagonists:

  • $H_1$-Receptor Antagonists: Block $H_1$ histamine receptors involved in smooth muscle contraction, vasodilation, vascular permeability, and irritant receptor stimulation. Treats edema, inflammation, itching (pruritus), rash, rhinorrhea (runny nose), red/watery eyes, and sneezing.

  • $H_2$-Receptor Antagonists: Block $H_2$ receptors involved in gastric acid secretion (e.g., Famotidine).

• Antihistamine Generations:

  • First-Generation Antihistamines:

  • Diphenhydramine (Benadryl).

  • Routes: Oral (PO), Intramuscular (IM), Intravenous (IV).

  • Properties: Crosses the blood-brain barrier causing significant sedation; also effective for motion sickness.

  • Second-Generation Antihistamines:

  • Cetirizine (Zyrtec), Loratadine (Claritin), Fexofenadine (Allegra).

  • Route: Oral (PO).

  • Properties: Minimal central nervous system penetration; non-drowsy formulation for 24-hour relief of indoor and outdoor allergies.

Epinephrine (Adrenaline) Therapeutics in Anaphylaxis

• Pharmacological Classification:

  • Adrenergic agonist, sympathomimetic agent.

  • Non-selective agonist acting on both alpha ($\alpha$) and beta ($\beta$) adrenergic receptors.

• Mechanism of Action:

  • Directly halts the release of inflammatory mediators from mast cells and basophils.

  • $\alpha_1$-Adrenergic Activation: Mediates potent peripheral vasoconstriction, reversing systemic vasodilation, raising blood pressure, and reducing mucosal edema.

  • $\beta_1$-Adrenergic Activation: Increases cardiac output, heart rate, and myocardial contractility.

  • $\beta_2$-Adrenergic Activation: Induces bronchial smooth muscle relaxation (bronchodilation), reversing life-threatening airway obstruction.

• Clinical Indications:

  • First-line treatment for severe allergic emergency (anaphylaxis).

  • Cardiac arrest.

  • Severe acute asthma attacks.

• Major Adverse Effects:

  • Tachycardia, hypertension, cardiac dysrhythmias, restlessness, anxiety, tremor.

• Dosing and Concentration Specifications:

  • $1:1000$ Concentration ($1\,\text{mg/mL}$ ): Used for Intramuscular (IM) injection.

  • Adult Anaphylaxis Dose: $0.2\text{--}1\,\text{mg}$ IM (standard auto-injector dose $0.3\,\text{mg}$; maximum single dose $0.5\,\text{mg}$).

  • Pediatric Anaphylaxis Dose: $0.01\,\text{mg/kg}$ IM (standard Jr auto-injector dose $0.15\,\text{mg}$).

  • $1:10,000$ Concentration ($1\,\text{mg in } 10\,\text{mL}$): Used for Intravenous (IV) push in cardiac arrest.

  • Packaging Preparations: $1\,\text{mg/mL}$ single-use vial ($1:1000$), $30\,\text{mL}$ multi-dose vial ($1:1000$), $1\,\text{mg in } 10\,\text{mL}$ prefilled syringe ($1:10,000$).

• Routes of Administration:

  • Intramuscular (IM): Preferred, most effective route for anaphylaxis (injected into outer mid-thigh).

  • Subcutaneous (SQ).

  • Intravenous (IV): Preferred route in cardiac arrest or severe refactory shock.

  • Inhalation / Nasal Spray: Neffy ($2\,\text{mg}$ single-dose nasal spray device) / Nebulized epinephrine.

  • Endotracheal (ET) Tube.

• EpiPen Auto-Injector Specifications and Step-by-Step Instructions:

  • Formulations: EpiPen Adult ($0.3\,\text{mg}$, $1:1000$) vs. EpiPen Jr ($0.15\,\text{mg}$, $1:2000$).

  • Physical Device Dimensions: Length $15\,\text{cm}$, Width $2.7\,\text{cm}$.

  • Device Anatomy: Features a blue safety release cap, adrenaline viewing window (must inspect solution; replace unit if solution is discolored or precipitate is present), and an orange needle-guard end.

• Step-by-Step Administration Steps:

  1. Form a fist around the EpiPen with the blue cap pointing upward ("Blue to the sky, orange to the thigh").

  2. Pull the blue safety cap straight upward to remove.

  3. Place the orange tip against the outer mid-thigh at a $90^\circ$ angle. (Can inject directly through clothing if required; do not press down yet).

  4. Swing and push the auto-injector firmly into the outer thigh until a distinct "click" is heard.

  5. Hold firmly in place against the thigh and count slowly to 3 ($1... 2... 3...$ seconds).

  6. Remove the EpiPen straight out from the thigh. Gently massage the injection area for $10\,\text{seconds}$. Call emergency medical services (911) immediately.

• Nursing Considerations and Patient Education:

  • MONITOR CLOSELY: Continuous monitoring of vital signs, airway patency, and reversal of symptoms (watch closely for rebound anaphylaxis as epinephrine wears off).

  • Dosage Verification: Epinephrine overdose can be fatal due to severe hypertension or cardiac dysrhythmias.

  • Patient Teaching: Always take exactly as directed. Patients must seek immediate emergency medical evaluation after administering a dose because the causative allergen can persist in the body much longer than the therapeutic duration of epinephrine.

Autoimmunity & Immunological Tolerance

• Definition of Autoimmunity:

  • An abnormal, destructive immune response directed against host self-cells and tissue components.

• Mechanisms of Immunological Tolerance:

  • Central Tolerance:

  • T and B lymphocytes express receptors that recognize host "self" proteins.

  • Immature self-reactive lymphocytes are deleted (eliminated via apoptosis / negative selection) during early development in primary lymphoid organs (thymus for T-cells, bone marrow for B-cells).

  • Peripheral Tolerance (Anergy):

  • Serves as a backup mechanism if self-reactive lymphocytes escape central elimination.

  • Self-reactive lymphocytes in peripheral tissues become functionally inactivated (anergic) or suppressed by regulatory T-cells.

  • Failure of Tolerance: Simultaneous breakdown of central and peripheral tolerance mechanisms results in autoimmune disease, leading to tissue destruction and organ failure.

• Specific Cellular and Receptor Autoimmune Targets:

  • Attacking Specific Cells: Destruction of pancreatic $\beta$-cells in Type 1 Diabetes Mellitus (T1DM).

  • Attacking Cell Receptors: Autoantibody destruction/blockade of nicotinic acetylcholine receptors (anti-AChR antibodies) at the neuromuscular junction in Myasthenia Gravis.

• Etiology and Pathogenesis:

  • Multifactorial Etiology: Combination of genetic predisposition and environmental triggers.

  • Antigenic Mimicry:

  • Certain bacterial or viral pathogens express antigens with structural sequences that closely resemble host self-antigens.

  • Following infection, sensitized immune cells cross-react with self-antigens, unable to distinguish between foreign pathogens and host tissues.

• Recognized Risk Factors:

  • Sex: Women have a significantly higher incidence of autoimmune diseases.

  • Age: Increased incidence linked to immunosenescence (the gradual decline and dysregulation of immune function associated with aging).

  • Ethnicity: Specific genetic lineages display heightened susceptibility.

  • Infection: Prior viral or bacterial exposures.

  • Obesity: Chronic low-grade systemic inflammation promoting immune dysregulation.

• Pharmacological Treatment Options & Clinical Balance:

  • Pharmacotherapy: Administer immunosuppressive agents, systemic corticosteroids, or cytotoxic chemotherapeutics to stop or slow the hyperactive immune response.

  • Clinical Balance: Therapy requires precise titration to achieve a delicate balance between controlling destructive autoimmune inflammation and avoiding catastrophic immunocompromise (which leaves the patient vulnerable to severe infection).