Hypersensitivity

Basic Principles of Hypersensitivity & Lymphocyte Function

• Definition of Hypersensitivity

  • Hypersensitivity represents a normal immune response that is inappropriately triggered, excessive, and produces undesirable or damaging effects on the body.

  • It is an immune reaction directed against a harmless molecule encountered in the environment.

  • Initial contact sensitizes the immune system; subsequent re-exposure to the antigen triggers an inflammatory reaction.

• Antigens and Lymphocytes Overview

  • Antigens

    • Structures recognized by the immune system as foreign.

    • Composed chemically of proteins or carbohydrates.

    • Trigger an adaptive immune response.

  • B-Lymphocytes (B-Cells)

    • Produce specialized antibodies, or immunoglobulins, targeting specific antigens.

    • Antibodies bind to specific antigens to neutralize or destroy them.

  • T-Lymphocytes (T-Cells)

    • Express T-cell receptors that recognize specific antigens.

    • Cytotoxic T-Cells ($CD8$): Exert direct toxic effects on specific target cells.

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

• Hypersensitivity Triggers and Classification

  • Basic Triggers

    • Antigen reacts with circulating antibodies (antigen-antibody reaction).

    • Antigen reacts with T-lymphocytes directly (antigen-lymphocyte reaction, which operates at a slower rate).

  • Classifications Overview

    • Type I Hypersensitivity: Antibody-mediated (IgE-driven).

    • Type II Hypersensitivity: Antibody-mediated (IgG/IgM-driven cytotoxic response against cell-bound antigens).

    • Type III Hypersensitivity: Antibody-mediated (IgG/IgM-driven immune complex deposition).

    • Type IV Hypersensitivity: T-cell-mediated (delayed cellular response, independent of antibodies).

Type I Hypersensitivity: IgE-Mediated Reactions

• General Characteristics

  • Produces an immediate reaction, occurring within $15 - 20$\,minutes of antigen exposure.

  • Requires prior sensitization to the antigen.

  • First exposure creates antibody production without overt clinical symptoms; subsequent re-exposure triggers the immediate inflammatory cascade.

• Common Antigens

  • Environmental: Animal dander, bee venom, dust, mold, pollen.

  • Foods: Nuts, seafood, eggs.

  • Medications: Penicillin, radiocontrast dye.

• Etiology and Genetic Risk Factors

  • Strong genetic predisposition:

    • Child with $1$ allergic parent: $2\times - 3\times$ increased risk.

    • Child with $2$ allergic parents: $5\times - 10\times$ increased risk.

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

  • Key cellular components: B-lymphocytes, Plasma cells, IgE immunoglobulins, Mast cells.

• Pathogenesis Cascade

  1. Exposure to an environmental antigen.

  2. Antigen is processed and presented to helper T-cells ($CD4$).

  3. Helper T-cells stimulate antigen-specific B-lymphocytes.

  4. B-lymphocytes undergo activation and mature into antibody-secreting plasma cells.

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

  6. IgE antibodies bind via their Fc region to high-affinity receptors on surface mast cells.

  7. Re-exposure to the specific antigen leads to antigen binding and cross-linking of adjacent IgE antibodies on the mast cell membrane.

  8. Cross-linking triggers immediate mast cell degranulation.

  9. Mast cells dump intracellular granules—primarily histamine—into surrounding tissues, driving clinical manifestations.

• Histamine Pathophysiology and Systemic Effects

  • Intravascular: Severe vasodilation and fluid shifts leading to anaphylactic shock.

  • Dermatological: Urticaria (hives), angioedema (deep dermal/subcutaneous swelling), atopic dermatitis, and classic wheal-and-flare cutaneous reactions.

  • Respiratory: Rhinitis (nasal mucosa inflammation, congestion, rhinorrhea) and bronchospasm (asthma).

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

• Mediator Activity & Manifestation Correlation

  • Potent Vasodilation: Causes stuffy nose, lowered blood pressure, compensatory increased heart rate, and raised skin wheals.

  • Increased Vascular Permeability: Causes endothelial fluid leakage resulting in tissue edema and rhinorrhea.

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

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

• Localized vs. Systemic Manifestations

  • Localized (Atopic Reactions): Confined to specific tissues; includes allergic rhinitis, asthma, and localized urticaria. Primary triggers include pollen, dust, mold, and animal dander.

  • Systemic (Anaphylaxis): Massive systemic release of chemical mediators causing diffuse bronchial constriction, complete airway obstruction, and severe vascular collapse (shock). Primary triggers include injected medications, bee venom, and ingested foods.

Type II Hypersensitivity: Cytotoxic & Antibody-Mediated Reactions

• General Mechanisms

  • Cell-surface markers (antigens) stimulate the production of specific target antibodies (IgG or IgM).

  • Antibodies bind directly to surface antigens present on body cells or transfused cells.

  • Binding leads to direct cell destruction via complement-mediated cell lysis, phagocytosis, or antibody-dependent cellular cytotoxicity (ADCC) mediated by cytotoxic T-cells, natural killer (NK) cells, macrophages, and neutrophils.

• Target Antigens & Clinical Conditions

  • Exogenous antigens: Transfused foreign blood cells.

  • Endogenous autoantigens: Body's own cellular membrane components.

  • Associated diseases: Autoimmune hemolytic anemia, Myasthenia Gravis, Erythroblastosis Fetalis.

• Erythroblastosis Fetalis Pathophysiology

  • Conception occurs between an $Rh\text{-negative}$ woman and an $Rh\text{-positive}$ man, resulting in an $Rh\text{-positive}$ fetus.

  • Fetal $Rh\text{-positive}$ red blood cells enter maternal circulation during gestation or delivery.

  • The $Rh\text{-negative}$ mother becomes sensitized and produces anti-Rh antibodies (IgG).

  • In a subsequent pregnancy with an $Rh\text{-positive}$ fetus, maternal IgG anti-Rh antibodies cross the placenta and attack fetal red blood cells, causing severe fetal hemolysis.

• Immunohematology & Transfusion Rules

  • Blood Group Compatibility:

    • Type A+: Can receive $A+$, $A-$, $O+$, $O-$.

    • Type A-: Can receive $A-$, $O-$.

    • Type B+: Can receive $B+$, $B-$, $O+$, $O-$.

    • Type B-: Can receive $B-$, $O-$.

    • Type AB+: Universal Recipient (can receive all ABO/Rh types).

    • Type AB-: Can receive all negative blood types ($A-$, $B-$, $AB-$, $O-$).

    • Type O+: Can donate to all positive blood groups ($A+$, $B+$, $AB+$, $O+$).

    • Type O-: Universal Donor (can donate to all groups; utilized in emergency transfusions).

  • Rh Factor System:

    • $Rh+$: Expresses the surface D antigen.

    • $Rh-$: Devoid of the surface D antigen.

  • Transfusion Mismatch Example:

    • Type A donor blood (bearing A antigens) is transfused into a Type B recipient.

    • Pre-existing anti-A antibodies in the Type B recipient's plasma immediately bind to donor Type A red blood cells.

    • Bound anti-A antibodies trigger the complement cascade, causing rapid intravenous hemolysis and systemic release of free hemoglobin.

  • Clinical Imperatives: Mandatory cross-matching prior to blood administration to prevent catastrophic hemolytic reactions.

Complement Cascade Mechanisms & Hemolytic Transfusion Reactions

• Classical vs. Alternative Complement Activation

  • Classical Pathway: Initiated by antigen-antibody complexing (specifically bound IgG or IgM).

  • Alternative Pathway: Initiated by direct surface pathogen markers.

• Molecular Cascade Steps

  1. Pathway activation leads to assembly of the $C1$ complex.

  2. Cleavage of downstream complement components produces $C2a$ and $C4b$ fragments.

  3. $C2a$ and $C4b$ combine to form $C3$ convertase.

  4. $C3$ convertase mediates $C3$ hydrolysis into $C3a$ (anaphylatoxin mediator) and $C3b$ (opsonin fragment).

  5. $C3b$ cleaves $C5$ into $C5a$ (inflammatory mediator) and $C5b$.

  6. Component $C5b$ sequentially recruits $C6$, $C7$, $C8$, and multiple $C9$ molecules.

  7. The assembled $C5b-C9$ complex forms the cylindrical Membrane Attack Complex (MAC).

  8. MAC translocates into the target cell lipid bilayer, creating a transmembrane pore.

  9. Uncontrolled osmotic water influx causes the target cell to swell and undergo osmotic lysis.

• Type II Clinical Manifestations

  • Cutaneous: Urticaria (hives), pruritus, peripheral edema.

  • Respiratory: Wheezing, shortness of breath, chest tightness.

  • Gastrointestinal: Nausea, vomiting, diarrhea.

Type III Hypersensitivity: Immune Complex-Mediated Reactions

• Pathophysiologic Mechanisms

  • Formed by soluble antigens binding to circulating antibodies (IgG or IgM) forming antigen-antibody complexes.

  • Complexes fail to be cleared effectively by the reticuloendothelial system and deposit directly into tissue beds (such as vascular basement membranes, renal glomeruli, and joint synovium).

  • Deposited complexes activate the complement cascade, generating potent chemotactic factors ($C3a$, $C5a$) that attract neutrophils and mast cells.

  • Neutrophils attempt to phagocytose complexes and release lysosomal enzymes, causing structural endothelial and basement membrane destruction.

• Etiological Antigens

  • Foreign Antigens: Bacterial or viral proteins, therapeutic drugs, foreign antivenoms, vaccines.

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

• Prototypical Clinical Examples

  • Post-streptococcal glomerulonephritis.

  • Rheumatoid arthritis.

  • Systemic lupus erythematosus (SLE).

• Clinical Manifestations

  • Symptoms depend on the specific anatomical site of complex deposition and tissue injury.

Type IV Hypersensitivity: Delayed & Cell-Mediated Reactions

• General Characteristics

  • Antibody-Independent: Mediated entirely by cellular immune mechanisms (T-cells).

  • Delayed Response: Clinical onset is delayed (typically $24 - 72$\,hours) due to the required time frame for T-cell recruitment, antigen recognition, and clonal expansion.

• Etiological Antigens

  • Plant catechol oils (poison ivy, poison oak).

  • Cosmetics, dyes, adhesives, clothing chemicals.

  • Jellyfish toxins.

  • Nickel alloys and metallic compounds.

  • Tuberculin protein extract.

  • Dietary gluten.

  • Allogeneic transplanted organs or tissues.

• Pathogenesis and Cellular Dynamics

  1. Small, incomplete foreign molecules (haptens) penetrate the cutaneous barrier.

  2. Haptens covalently bind to endogenous carrier proteins to form a complete immunogenic antigen.

  3. Antigen-Presenting Cells (APCs, such as Langerhans cells) internalize, process, and present the hapten-protein complex via MHC molecules.

  4. APCs migrate to regional lymph nodes and present antigen to naive T-lymphocytes.

  5. Sensitized Helper T-cells ($T_H1$) and Cytotoxic T-cells ($CD8$ / $T_c$) undergo proliferation and generate Memory T-cells.

  6. Upon re-exposure, activated $T_H1$ cells secrete inflammatory cytokines.

  7. Cytokines recruit and activate macrophages, which release tissue-destructive lysosomal enzymes and reactive oxygen species to clear cellular targets.

• Clinical Manifestations

  • Contact Dermatitis: Pruritic skin rash, localized erythema, and vesicular lesions.

  • Tuberculin Test Reaction: Localized induration (firm skin swelling) and erythema at the intradermal injection site.

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

  • Neurological Deficits: Occur when sensitized autoreactive T-cells attack central nervous system myelin sheaths (e.g., Multiple Sclerosis).

Comparative Matrix of Hypersensitivity Reaction Types

• Type I (IgE-Mediated)

  • Immune Reactant: IgE antibodies.

  • Antigen Form: Soluble allergens.

  • Mechanism: Allergen-specific IgE binds mast cells via Fc receptors; cross-linking triggers mast cell degranulation.

  • Clinical Examples: Systemic anaphylaxis, seasonal hay fever (allergic rhinitis), food allergies, drug allergies.

• Type II (Cytotoxic / Antibody-Mediated)

  • Immune Reactant: IgG or IgM antibodies.

  • Antigen Form: Cell-bound or tissue-bound antigens.

  • Mechanism: Antibody binding induces complement-mediated lysis, phagocytosis, or ADCC via NK cells, T-cells, macrophages, and neutrophils.

  • Clinical Examples: Transfusion reactions from blood incompatibility, hemolytic disease of the newborn (erythroblastosis fetalis).

• Type III (Immune Complex-Mediated)

  • Immune Reactant: IgG and IgM antibodies.

  • Antigen Form: Soluble antigens.

  • Mechanism: Circulating antigen-antibody complexes deposit in tissue membranes, activating complement and attracting neutrophils that release tissue-damaging enzymes.

  • Clinical Examples: Post-streptococcal glomerulonephritis, rheumatoid arthritis, systemic lupus erythematosus.

• Type IV (Delayed / T-Cell-Mediated)

  • Immune Reactant: Sensitized T-lymphocytes ($CD4$ $T_H1$ and $CD8$ Cytotoxic T-cells).

  • Antigen Form: Soluble or cell-bound antigens/haptens.

  • Mechanism: $T_H1$ cells release cytokines that activate macrophages and cytotoxic T-cells, causing tissue damage.

  • Clinical Examples: Contact dermatitis, Type 1 Diabetes Mellitus, Multiple Sclerosis.

Pharmacotherapy for Hypersensitivity: Antihistamines & Immunosuppressants

• Broad Pharmacologic Strategies

  • Immunosuppressants: Dampen cell-mediated adaptive immune responses.

  • Anti-inflammatory Medications (Corticosteroids): Inhibit multi-pathway inflammatory cascades; carry an inherent risk of increased host vulnerability to systemic infection.

  • Antihistamines: Inhibit histamine release or block histamine target cell receptors.

  • Epinephrine: Rapidly halts mast cell mediator secretion; reserved strictly for acute anaphylaxis.

• Antihistamine Mechanisms of Action

  • Inhibition of Mediator Release / Mast Cell Stabilization:

    • Cromolyn Sodium: Inhibits mast cell degranulation; highly effective in asthma and chronic obstructive pulmonary disease (COPD).

    • Leukotriene Modifiers: Inhibit leukotriene-mediated inflammatory pathways.

  • Receptor Antagonism:

    • $H_1$ Receptor Antagonists: Block $H_1$ receptors involved in capillary permeability, vasodilation, itching, and smooth muscle constriction.

    • $H_2$ Receptor Antagonists: Block $H_2$ receptors involved in parietal cell gastric acid secretion.

  • Treated Symptoms: Edema, inflammation, pruritus, skin rashes, rhinorrhea, conjunctival redness/lacrimation, sneezing.

• Antihistamine Classifications

  • First-Generation $H_1$ Antagonists:

    • Prototype: Diphenhydramine.

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

    • Characteristics: Highly lipophilic; crosses the blood-brain barrier resulting in marked central nervous system sedation (primarily in adult populations); clinically utilized for motion sickness.

  • Second-Generation $H_1$ Antagonists:

    • Prototypes: Cetirizine, Loratadine, Fexofenadine.

    • Routes: Oral (PO).

    • Characteristics: Non-sedating; minimal central nervous system penetration.

Emergency Management: Epinephrine Pharmacology & Administration Guidelines

• Pharmacologic Classification & Receptor Dynamics

  • Classification: Non-selective adrenergic agonist, sympathomimetic agent.

  • Receptor Actions:

    • $\alpha_1$-Adrenergic Activation: Induces potent peripheral vascular smooth muscle constriction, elevating blood pressure and reducing airway mucosal edema.

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

    • $\beta_2$-Adrenergic Activation: Drives rapid bronchial smooth muscle relaxation (bronchodilation) and inhibits further inflammatory mediator release from mast cells.

• Clinical Indications

  • Severe Type I allergic reactions (Anaphylaxis).

  • Cardiac arrest (stimulates cardiac electrical and mechanical activity).

  • Severe acute asthma exacerbations.

• Adverse Effects

  • Tachycardia, severe hypertension, cardiac dysrhythmias, marked anxiety/restlessness, peripheral tremors.

• Dosing & Administration Concentrations

  • Standard IM Concentration: $1:1000$ ($1\,\text{mg/mL}$).

  • Intravenous Cardiac Concentration: $1:10,000$ ($1\,\text{mg in } 10\,\text{mL}$).

  • Pediatric Dosage: $0.01\,\text{mg/kg}$.

  • Adult Dosage: $0.2 - 1\,\text{mg}$ (maximum single dose $0.5\,\text{mg}$ IM/SQ).

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

  • Administration Routes: Subcutaneous (SQ), Intramuscular (IM — preferred route for anaphylaxis), Intravenous (IV — preferred route for cardiac arrest), Inhalation / Nebulized / Nasal spray (Neffy), Endotracheal (ET) tube.

• Nursing Considerations & Patient Safety

  • Monitoring: Continuous assessment of vital signs, airway patency, and symptom resolution.

  • Biphasic Risk Warning: Epinephrine is rapidly metabolized by the body; its therapeutic duration may be shorter than the half-life of the triggering allergen, leading to a recurrence of anaphylaxis symptoms.

  • Dose Calculation: Exact verification is mandatory; accidental overdose can be fatal.

  • Patient Teaching: Instruct patients to carry auto-injectors at all times, administer strictly as directed, seek immediate emergency medical evaluation (call 911), and follow up with a healthcare provider post-administration.

• Step-by-Step EpiPen Auto-Injector Administration Procedure

  1. Form a firm grip around the EpiPen with the dominant fist, keeping the blue safety cap pointing straight up ("BLUE TO THE SKY").

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

  3. Position the orange tip against the outer middle thigh at a perpendicular $90$-degree angle ("ORANGE TO THE THIGH"). Do not depress the tip yet (can inject directly through clothing if necessary).

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

  5. Hold the auto-injector firmly in place against the thigh while counting slowly to $3$\,seconds ($1 \dots 2 \dots 3$).

  6. Remove the device straight out from the thigh.

  7. Gently massage the injection site for $10$\,seconds.

  8. Call 911 immediately.

  • Standard Auto-Injector Dose: $0.3\,\text{mg}$ epinephrine.

  • EpiPen 2-Pak Contents: Contains two yellow active $0.3\,\text{mg}$ auto-injectors and one grey needleless/drug-free trainer pen.

Autoimmunity: Mechanisms, Etiology, & Clinical Management

• Definition & Tolerance Mechanisms

  • Autoimmunity: An abnormal and destructive immune response targeted against the body's own ("self") healthy tissues.

  • Central Immunological Tolerance:

    • Occurs during primary lymphocyte maturation (thymus for T-cells, bone marrow for B-cells).

    • Immature T and B lymphocytes that express high-affinity receptors for self-proteins are recognized and eliminated (deleted) within primary lymphoid tissues.

  • Peripheral Immunological Tolerance (Anergy):

    • Functional inactivation (anergy) of self-reactive lymphocytes that escape central tolerance mechanisms, preventing auto-reactivity in peripheral tissues.

  • Disease Onset: Occurs when both central and peripheral immune tolerance mechanisms fail, leading to progressive tissue and organ destruction.

• Pathophysiologic Targets

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

  • Specific Membrane Receptors: Blockade and destruction of post-synaptic acetylcholine receptors by anti-AChR autoantibodies in Myasthenia Gravis.

• Etiology and Risk Factors

  • Complex Etiology: Multifactorial interaction of genetic susceptibility and environmental triggers.

  • Antigenic Mimicry:

    • Exogenous bacterial or viral antigens share structural epitope similarity with host tissue self-antigens.

    • Activated immune cells fail to distinguish between the invading pathogen and host self-antigens, mounting a cross-reactive immune attack against host tissues following an infection.

  • Identified Risk Factors:

    • Sex: Females are $2\times - 3\times$ more frequently affected than males.

    • Age: Increased susceptibility associated with immunosenescence (the progressive age-related decline in immune regulation).

    • Ethnicity: Elevated incidence in African American and Asian populations.

    • Environmental: Recent viral or bacterial infections, obesity.

• Pharmacologic Management

  • Therapeutic Goal: Halt or suppress autoreactive immune responses using systemic immunosuppressants, synthetic corticosteroids, or cytotoxic chemotherapy agents.

  • Clinical Challenge: Requires maintaining a fine balance between adequate autoimmune suppression and minimizing host susceptibility to severe opportunistic infections.