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First Pass Metabolism (First Pass Effect)

Definition

"If the drug is rapidly metabolized in the liver or gut wall during this initial passage, the amount of unchanged drug entering the systemic circulation is decreased." - Lippincott Illustrated Reviews: Pharmacology

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How It Happens - Step by Step

1. Drug is swallowed and absorbed through the intestinal wall into mesenteric capillaries
2. These drain into the portal vein, which goes directly to the liver
3. Hepatic enzymes (primarily CYP450 enzymes, especially CYP3A4) metabolize a portion of the drug
4. The gut wall also contributes (e.g., CYP3A4 and P-glycoprotein efflux)
5. Only the fraction that survives this first passage enters the hepatic vein → inferior vena cava → systemic circulation

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Effect on Bioavailability

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Classic Examples

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Sites of First Pass Metabolism

• Gut wall - CYP3A4 and P-glycoprotein (e.g., benzylpenicillin, insulin degraded here)
• Liver - Main site; CYP450 enzymes (propranolol, lidocaine, nitroglycerin, morphine)
• Lung - For drugs given by non-GI parenteral routes; can serve as a site of first pass loss via excretion or metabolism

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Routes That Bypass First Pass Metabolism

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Clinical Relevance

1. Dose adjustment: Drugs with extensive first pass need much higher oral doses than IV doses (e.g., morphine oral:IV ratio is approximately 3:1)
2. Inter-patient variability: Differences in CYP450 activity, hepatic blood flow, and genetic polymorphisms cause wide variation between individuals
3. Liver disease: Cirrhosis with portosystemic shunting can dramatically increase bioavailability of high first-pass drugs (e.g., morphine, meperidine, midazolam, nifedipine oral bioavailability nearly doubles in cirrhosis - per Harrison's Principles of Internal Medicine)
4. Drug interactions: One drug can inhibit or induce the enzymes involved, affecting the first pass metabolism of another drug (e.g., grapefruit juice inhibits CYP3A4)
5. Prodrugs: Some drugs are designed to exploit first pass - they are inactive until metabolized by the liver (e.g., codeine → morphine via CYP2D6; enalapril → enalaprilat)

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Summary Formula

Oral bioavailability (F) = Fraction absorbed × (1 - Extraction Ratio)

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NOTE: Drug Absorption & Factors Influencing It

What is Drug Absorption?

"Absorption from the site of administration permits entry of the drug (either directly or indirectly) into plasma." - Lippincott Illustrated Reviews: Pharmacology

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Mechanisms of Drug Absorption Across Membranes

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Factors Influencing Drug Absorption

1. pH and Drug Ionization (pKa)

• Un-ionized form = lipid soluble → crosses membranes freely
• Ionized form = water soluble, charged → does NOT cross lipid membranes well

• Weak acids (e.g., aspirin, warfarin): better absorbed in acidic environments (stomach) where they are un-ionized
• Weak bases (e.g., morphine, codeine, atropine): better absorbed in alkaline environments (small intestine) where they are un-ionized

Ion trapping: When a drug passes into a compartment where it becomes ionized, it gets "trapped" there - it cannot diffuse back out. This is exploited clinically, e.g., alkalinizing urine to trap acidic drugs like aspirin for faster excretion in overdose.

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2. Lipid Solubility

• Drugs must be lipophilic enough to dissolve in the lipid bilayer and cross membranes
• But must also have some water solubility to dissolve in GI fluids and reach the membrane surface
• Extremely hydrophilic OR extremely lipophilic drugs are both poorly absorbed
• This is why many drugs are weak acids or weak bases - they have partial lipid solubility

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3. Route of Administration

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4. GI Motility and Gastric Emptying

• Increased GI motility (e.g., diarrhea, metoclopramide) → faster gastric emptying → drug reaches small intestine sooner → may increase absorption rate but reduce total time for absorption
• Decreased motility (e.g., opioids, anticholinergics) → delayed gastric emptying → slower absorption onset
• Most oral absorption occurs in the small intestine (large surface area, rich vascular supply), so faster gastric emptying generally increases drug absorption

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5. Blood Flow to the Absorption Site

• Greater blood flow maintains a high concentration gradient across the membrane, driving faster absorption
• IM injections absorb faster than SC because muscle has richer blood supply
• In shock states, peripheral absorption (IM, SC) is unreliable due to vasoconstriction - IV is preferred
• Sustained-release and depot preparations slow drug release to prolong absorption regardless of blood flow

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6. Solubility and Drug Formulation

"Drug absorption may be altered by factors unrelated to the chemistry of the drug. For example, particle size, salt form, crystal polymorphism, enteric coatings, and the presence of excipients (such as binders and dispersing agents) can influence the ease of dissolution and absorption." - Lippincott Pharmacology

• Particle size: smaller particles dissolve faster (more surface area)
• Salt form: e.g., sodium salts dissolve faster than free acid forms
• Crystal polymorphism: different crystal forms of the same drug dissolve at different rates
• Enteric coatings: protect acid-labile drugs (e.g., omeprazole) or irritating drugs (e.g., aspirin EC) - release occurs in the intestine
• Extended-release (ER/SR/XL) formulations: rate of dissolution controls rate of absorption; allows twice or once-daily dosing

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7. Food and Drug Interactions

• Food can delay gastric emptying, dilute drug concentration, or chelate certain drugs (e.g., tetracyclines bind calcium in dairy)
• P-glycoprotein (P-gp): an efflux transporter in the gut wall that pumps some drugs back into the intestinal lumen, reducing absorption (e.g., digoxin). Inhibitors like verapamil increase digoxin bioavailability
• Grapefruit juice: inhibits intestinal CYP3A4 → increases absorption of many drugs (e.g., statins, calcium channel blockers)
• Antacids: raise gastric pH, altering ionization of drugs (e.g., reduces absorption of ketoconazole which needs acidic environment)

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8. Drug Chemical Instability

• Some drugs are degraded before absorption:
  • Penicillin G: unstable in acidic gastric environment
  • Insulin: proteolytic enzymes in the GI tract destroy it completely - must be given by injection
  • Nitroglycerin: destroyed by hepatic first pass (>90%) if swallowed

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9. Patient-Specific Factors

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Summary Diagram of Key Concepts

Drug administered orally
        ↓
Dissolved in GI fluids (solubility)
        ↓
Un-ionized form (depends on pKa + pH) crosses gut wall
        ↓
P-gp efflux? CYP3A4 in gut wall? → reduced absorption
        ↓
Portal vein → LIVER → first pass metabolism
        ↓
Remaining drug → systemic circulation = BIOAVAILABILITY (F)

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The Henderson-Hasselbalch Principle in Pharmacology

Background: Why It Matters

• Un-ionized (uncharged) form - lipid soluble, crosses membranes freely
• Ionized (charged) form - water soluble, CANNOT cross lipid membranes

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The Equation - Derivation

"The lower the pH relative to the pKa, the greater will be the fraction of drug in the protonated form." - Katzung's Basic & Clinical Pharmacology

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Applying the Equation to Drugs

For Weak ACIDS (e.g., aspirin, warfarin, furosemide):

• Acidic environment (low pH, stomach pH ~1-3): More drug stays as HA (un-ionized) → better absorbed
• Alkaline environment (high pH, intestine pH ~7.5-8): More drug becomes A⁻ (ionized) → less absorbed from that site

For Weak BASES (e.g., morphine, codeine, atropine, amphetamine):

• Alkaline environment (intestine, blood pH 7.4): More drug stays as B (un-ionized) → better absorbed
• Acidic environment (stomach): More drug becomes BH⁺ (ionized) → less absorbed

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The Key Rule (Memory Aid)

"Acids are absorbed in Acid; Bases are absorbed in Base"

Or more precisely: un-ionized form = absorbed form

• Weak acid: un-ionized in acidic environment
• Weak base: un-ionized in alkaline environment

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The Golden Rule: When pH = pKa

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Worked Example

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pKa Values of Common Drugs (Katzung)

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Clinical Application 1: Ion Trapping

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Clinical Application 2: Urine Alkalinization / Acidification in Overdose

Rule: Weak acids are excreted faster in alkaline urine; weak bases are excreted faster in acidic urine

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Clinical Application 3: pH Compartments in the Body

Note: Basic drugs (e.g., some antibiotics, opioids) tend to concentrate in breast milk (pH ~6.4-7.6) because the slightly acidic-to-neutral pH traps them in ionized form - important for drug safety in breastfeeding mothers.

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Summary

Henderson-Hasselbalch:  pH = pKa + log([ionized]/[un-ionized])

                        Un-ionized form = crosses membranes = absorbed
                        Ionized form    = water soluble = trapped/excreted

Weak ACID:   acid environment (low pH) → un-ionized → absorbed
Weak BASE:   basic environment (high pH) → un-ionized → absorbed

When pH = pKa → 50:50 ionized:un-ionized
Each unit pH above pKa (for acids) → 10x more ionized
Each unit pH below pKa (for bases) → 10x more ionized

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Katzung's Basic and Clinical Pharmacology, 16th Edition

What Is It?

• Bertram G. Katzung - MD, PhD; received his MD from State University of New York (1957) and PhD in Pharmacology from the University of California. He built the book from the ground up over decades.
• Todd W. Vanderah - co-author on the current edition, professor of pharmacology

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What Is Pharmacology? (As Defined by Katzung)

"Pharmacology can be defined as the study of substances that interact with living systems through chemical processes. These interactions usually occur by binding of the substance to regulatory molecules and activating or inhibiting normal body processes."

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Structure & Organization (13 Blocks / Chapters)

SECTION I - Basic Principles

SECTION II - Autonomic Drugs

SECTION III - Cardiovascular & Renal Drugs

SECTION IV - CNS Drugs

SECTION V - Drugs with Important Actions on Smooth Muscle

SECTION VI - Endocrine Drugs

SECTION VII - Chemotherapeutic Drugs

SECTION VIII - Special Topics

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Key Features of the Book

1. Drug Groups & Prototypes Approach

"Almost all the several thousand drugs currently available can be arranged into about 70 groups... For most groups, one or two prototype drugs can be identified that typify the most important characteristics."

2. Case Studies

3. Pharmacokinetic Principles (Chapter 3 Highlights)

4. Biotransformation (Chapter 4)

• Phase I reactions: Oxidation, reduction, hydrolysis via CYP450 enzymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4)
• Phase II reactions: Glucuronidation, sulfation, acetylation, glutathione conjugation
• Enzyme induction vs inhibition and clinical drug-drug interactions
• Genetic polymorphisms: Poor vs. extensive metabolizers (e.g., CYP2D6 for codeine)
• Gut microbiota as a site of drug metabolism

5. Pharmacogenomics (Chapter 5)

• CYP2C9 and warfarin dosing
• VKORC1 variants
• TPMT and thiopurine toxicity
• Transporter gene variations affecting drug absorption and excretion

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Why It's Uniquely Valuable

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The Book's Approach to Drug-Body Interactions

PHARMACOKINETICS = What the BODY does to the DRUG
    Absorption → Distribution → Metabolism → Elimination

PHARMACODYNAMICS = What the DRUG does to the BODY
    Receptor binding → Signal transduction → Physiologic effect → Dose-Response

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Short Note: SUPERANTIGENS

Definition

"Superantigens are proteins that are capable of stimulating large populations of T cells in a manner unrestricted by the class II major histocompatibility complex (MHC)." - Goldman-Cecil Medicine

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Normal Antigen Presentation vs. Superantigen Action

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Mechanism of Action

1. Superantigen is produced at an infected site and absorbed into the circulation
2. It binds to MHC class II molecules on APCs (macrophages, dendritic cells) - but OUTSIDE the normal peptide-binding groove
3. Simultaneously, it binds to the Vβ region of the T-cell receptor, independent of antigen specificity
4. This cross-links APC and T cell, activating them without any need for antigen processing
5. Up to 20% of the entire circulating T-cell pool is activated simultaneously
6. Massive release of cytokines: IL-1, IL-2, TNF-α, TNF-β, IFN-γ
7. This cytokine storm causes systemic effects - fever, hypotension, shock, organ failure

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Cytokine Cascade

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Important Superantigens and Associated Diseases

Staphylococcal Superantigens

Key fact: Staphylococcal enterotoxins are stable to boiling and gastric enzymes - so ingesting preformed toxin in food causes vomiting even without live bacteria.

Only <20% of S. aureus strains produce any SAg.

Streptococcal Superantigens

Streptococcal SAgs also cause fever, rash (scarlet fever), T-cell proliferation, B-lymphocyte suppression, and heightened sensitivity to endotoxin. SpeB additionally has direct enzymatic activity - digesting tissue and extracellular matrix proteins.

Other Superantigens

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Clinical Consequences: The Cytokine Storm

Superantigen
     ↓
Massive T-cell activation (up to 20% of T-cell pool)
     ↓
IL-1, IL-2, TNF-α, TNF-β, IFN-γ flood released
     ↓
↑ Vascular permeability → fluid leak → hypotension
↑ Fever (pyrogenic cytokines)
↓ Blood pressure → SHOCK
Organ hypoperfusion → multiorgan failure

• Sudden high fever (>38.9°C)
• Diffuse macular erythroderma (sunburn-like rash)
• Desquamation (1-2 weeks later, especially palms/soles)
• Hypotension (systolic BP <90 mmHg)
• Multisystem involvement (≥3 organ systems)
• Negative blood cultures (toxin circulates, not bacteria)

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Why Superantigens Are Unique - Key Points

1. No antigen processing needed - bind directly to intact MHC II without proteolysis
2. Non-specific Vβ binding - one SAg activates ALL T cells sharing that Vβ segment, regardless of their normal antigen specificity
3. Bivalent bridge - simultaneously binds APC and T cell
4. Scale of activation - normal immune response activates 0.001-0.01% of T cells; SAgs activate up to 20%
5. Toxin-mediated disease - only the toxin needs to circulate, not the bacteria (explains negative blood cultures in TSS)
6. Post-activation anergy/deletion - after the massive activation, T cells bearing the stimulated Vβ segments undergo activation-induced cell death, causing transient immunosuppression

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Association with Other Diseases

• Kawasaki disease: substantial attention has focused on TSST-1 as a potential trigger via the superantigen mechanism
• Granulomatosis with polyangiitis: superantigens postulated to play a role in disease flares
• Nasal polyposis: the staphylococcal superantigen hypothesis proposes that S. aureus exotoxins foster type 2 inflammation and polyposis in Caucasian patients (Cummings Otolaryngology)

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Immunoglobulins - Definition, Classification, and Detailed Description

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DEFINITION

"Antibodies belong to the immunoglobulin family of proteins, which appear in quantity in serum and on the surfaces of B cells." - Sherris & Ryan's Medical Microbiology

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CLASSIFICATION OF IMMUNOGLOBULINS

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BASIC STRUCTURE OF AN IMMUNOGLOBULIN

Components:

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IgG - Immunoglobulin G

a) Diagram

        Antigen-binding sites
           ↙           ↘
    [VH-VL]           [VH-VL]
       |   \         /   |
      CH1   -S-S-S-S-   CH1     ← Fab region (×2)
       |     Hinge    |
      CH2 — — — — — CH2         ← Fc region begins
       |               |
      CH3 — — — — — CH3
    (Fc receptor & complement binding)

b) Structure

• Form: Monomer (H₂L₂)
• Heavy chain: γ (gamma) - 4 domains: VH, CH1, CH2, CH3
• Light chain: κ or λ - 2 domains: VL, CL
• Subclasses: IgG1, IgG2, IgG3, IgG4 (each with distinct γ chains)
• MW: 150,000 Da
• Half-life: ~21-23 days (longest of all Ig classes)
• Valence: 2 (bivalent)

c) Functions

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IgA - Immunoglobulin A

a) Diagram

      [VH-VL] — [VH-VL]
         ↓           ↓
     H-L - S-S - H-L   (MW 160,000 Da)

   Monomer1 — J chain — Monomer2
                ↕
        Secretory Component
   (protects from proteolytic degradation)

b) Structure

• Serum form: Monomer (MW 160,000 Da)
• Secretory form: Dimer - two monomers joined by J chain + secretory component (SC)
• Heavy chain: α (alpha)
• Subclasses: IgA1 (mainly monomer in serum), IgA2 (mainly polymeric at mucosal surfaces)
• Secretory component: Poly-Ig receptor fragment; added during transcytosis across epithelium; protects sIgA from proteolytic digestion in secretions
• Valence: 2 (monomer), 4 (dimer)
• Humans produce more IgA by total mass than any other Ig class

c) Functions

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IgM - Immunoglobulin M

a) Diagram

b) Structure

• Form: Pentamer (5 × H₂L₂ units + 1 J chain)
• Heavy chain: μ (mu) - 5 domains (VH + CH1-CH4, no hinge region)
• J chain: Small glycoprotein that joins the 5 monomers and stabilizes the pentamer
• MW: 900,000 Da (largest Ig)
• Valence: 10 (10 antigen-binding sites) - though functionally only 5 may be used due to steric hindrance
• Serum IgM: Pentamer
• Membrane IgM (mIgM): Monomer - serves as the B-cell antigen receptor (BCR)

c) Functions

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IgE - Immunoglobulin E

a) Diagram

       [VH-VL]       [VH-VL]
          ↓               ↓
     H chain            H chain
    (5 domains:
    VH, Cε1, Cε2, Cε3, Cε4)
         ↕
    Fc binds high-affinity FcεRI
    on mast cells and basophils

b) Structure

• Form: Monomer (H₂L₂)
• Heavy chain: ε (epsilon) - 5 constant domains (no hinge region; Cε1-Cε4 + VH)
• MW: 200,000 Da (heaviest Ig monomer - due to extra constant domain)
• Valence: 2
• Serum concentration: Extremely low - 0.004% of total serum Ig (10-400 ng/mL)
• Half-life in serum: ~2 days; but when bound to mast cells/basophils: weeks
• Key receptors:
  • FcεRI (high affinity) - on mast cells and basophils
  • FcεRII / CD23 (low affinity) - on eosinophils, macrophages, neutrophils, platelets, B cells

c) Functions

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COMPARATIVE SUMMARY TABLE

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6. NOTE ON MONOCLONAL ANTIBODIES AND THEIR APPLICATIONS

Definition

"All molecules of a monoclonal antibody are produced by the progeny of a single B cell clone, and all the molecules therefore have the same V region and bind to the same antigen." - Cellular and Molecular Immunology (Abbas)

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Production - The Hybridoma Technique (Kohler & Milstein, 1975)

Steps:

1. Immunize a mouse with the antigen of interest
2. Isolate spleen B cells (produce antibody, but mortal)
3. Fuse B cells with myeloma cells (immortal, antibody-negative) using polyethylene glycol
4. Select in HAT medium (Hypoxanthine-Aminopterin-Thymidine):
   • Unfused myeloma cells die (lack HGPRT for salvage pathway)
   • Unfused B cells die naturally
   • Only hybridomas survive (get HGPRT from B cell + immortality from myeloma)
5. Screen hybridoma clones for desired antibody specificity
6. Expand the positive clone → indefinite production of mAb

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Types of Therapeutic Monoclonal Antibodies

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Applications of Monoclonal Antibodies

1. Therapeutic Applications

2. Diagnostic Applications

• Pregnancy tests - detection of hCG using mAbs
• Immunoassays (ELISA, RIA) - detection of antigens/antibodies in blood and urine for infectious disease, cancer markers, hormone levels
• Flow cytometry - CD marker identification on lymphocytes (e.g., CD4:CD8 ratio in HIV)
• Immunohistochemistry - tumor typing from biopsy sections (identify tissue of origin)
• Lateral flow assays - rapid point-of-care tests (e.g., COVID-19 antigen tests)

3. Research Applications

• Cell phenotyping - identification of CD markers (CD4, CD8, CD19, etc.) defining cell populations
• Functional blocking - mAbs that block receptor function help define molecule roles
• Western blotting / flow cytometry / confocal microscopy - standard research tools
• Purification - affinity chromatography using mAbs to purify specific proteins

4. Other Applications

• Radioimmunotherapy - mAbs conjugated with radioisotopes (e.g., I-131) target cancer cells
• Antibody-drug conjugates (ADC) - mAbs carry cytotoxic drugs directly to tumor cells (e.g., Ado-trastuzumab emtansine)
• Bispecific antibodies - engineered to bind two different antigens simultaneously (e.g., Blinatumomab: anti-CD19 × anti-CD3 for B-cell leukemia)

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Advantages and Limitations

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IgD (Immunoglobulin D) - What Does It Do?

Basic Properties

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The Honest Answer: Its Role Is Largely Mysterious

"To date, IgD has not been assigned a specific biological role as a humoral antibody... the role as a receptor is generally accepted, even though the nature and purpose of the signal it transmits remains controversial." - Henry's Clinical Diagnosis & Management by Laboratory Methods

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Known and Proposed Functions

1. B-Cell Antigen Receptor (Main Known Role)

• Naive mature B cells express both mIgM and mIgD simultaneously on their surface
• The antigen-binding sites (variable regions) and light chains of both are identical - only the constant (heavy chain) regions differ
• When antigen binds to mIgD (or mIgM) on the B-cell surface, it initiates signaling for B-cell activation and clonal proliferation

"The membrane-bound IgD may serve as one of the receptors with which B cells bind antigen and are stimulated to undergo clonal proliferation." - Henry's Clinical Diagnosis

2. Regulation of B-Cell Activation and Tolerance

• Evidence suggests that IgM-bearing B cells can respond to T-independent antigens, but acquisition of IgD is required for B cells to respond to T-dependent antigens (those needing T-cell help)
• If IgD is selectively removed from IgD+IgM+ B cells, those cells become susceptible to tolerance induction rather than activation
• IgD is therefore thought to "turn on, turn off, or modulate" B-cell division or differentiation - essentially a rheostat for B-cell responsiveness

3. Mucosal / Respiratory Immune Defense (Recently Discovered)

• IgD-producing plasma cells are found in tonsils and upper respiratory tract tissues - not just in blood
• At these mucosal sites, secreted IgD binds to galectin-9 on basophils and mast cells
• This binding appears to enhance protective humoral responses against respiratory pathogens AND inhibit IgE-induced allergic reactions at those sites

"IgD-producing plasma cells are found in tonsils and tissues associated with the respiratory tract, where IgD binds to galectin-9 on basophils and mast cells to enhance protective humoral responses and inhibit IgE-induced allergic reactions." - Firestein & Kelley's Textbook of Rheumatology

4. B-Cell Maturation Marker

• IgD surface expression marks the transition from immature to mature naive B cell
• Newborns have a high proportion of IgD-bearing B cells
• As B cells differentiate into plasma cells or memory cells after antigen stimulation, surface IgD is downregulated and lost

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What IgD Does NOT Do

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IgD in Disease

Hyper-IgD Syndrome (HIDS)

• Rare autosomal recessive autoinflammatory disorder
• Caused by mutations in the mevalonate kinase gene (MVK) - an enzyme in the cholesterol/isoprenoid biosynthesis pathway
• Features: recurrent episodes of high fever + lymphadenopathy + abdominal pain starting in infancy
• Serum IgD is elevated (>14 mg/dL or >100 IU/mL) - but this is now considered a secondary phenomenon to the underlying inflammatory process, not the cause
• Present equally in males and females

IgD Multiple Myeloma

• Rare (1-2% of myelomas)
• Monoclonal IgD spike in serum
• Generally has worse prognosis than other myeloma types
• Often associated with λ light chains and Bence Jones proteinuria

Chronic Infections

• Serum IgD levels rise non-specifically during chronic infection, along with other immunoglobulins
• No specific disease has been linked uniquely to IgD elevation

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Summary

IgD is mostly a:

    ┌─────────────────────────────────────────────────┐
    │  MEMBRANE RECEPTOR on mature naive B cells      │
    │  Co-expressed with IgM (same V region, diff. C) │
    │  Required for T-dependent antigen responsiveness│
    │  Modulates B-cell activation/tolerance          │
    └─────────────────────────────────────────────────┘

With emerging evidence of a MUCOSAL ROLE:
    → Tonsils / upper airway IgD plasma cells
    → Binds galectin-9 on basophils/mast cells
    → Enhances antibacterial responses, inhibits IgE allergy

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Hereditary Spherocytosis (HS)

Definition

"Hereditary spherocytosis is an inherited disorder caused by intrinsic defects in the red cell membrane skeleton that render red cells spheroid, less deformable, and vulnerable to splenic sequestration and destruction." - Robbins & Cotran Pathologic Basis of Disease

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Epidemiology

• Most common inherited hemolytic anemia in Northern Europeans
• Prevalence: 1 in 2000-5000 persons of Northern European ancestry
• Much less common in those of African ancestry
• Found worldwide; affects all races

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Genetics / Inheritance

• Most mutations are "private" (unique to each family) - no selective evolutionary advantage
• Rare homozygous cases can cause fetal death or very severe neonatal hemolysis

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Pathogenesis

1. Spectrin - Ankyrin - Band 3 (vertical linkage)
2. Spectrin tail - Protein 4.1 - Glycophorin A (lateral linkage)

Defective Proteins in HS:

The Sequence of Events:

Membrane protein deficiency
        ↓
Destabilization of lipid bilayer
        ↓
Membrane fragments shed as RBC ages in circulation
        ↓
↓ Surface area : Volume ratio
        ↓
RBC adopts spherical shape (smallest volume for given surface)
        ↓
Spherocyte - rigid, non-deformable
        ↓
Trapped in splenic cords (narrow sinusoids)
        ↓
Hostile splenic environment: ↓ glucose, ↓ pH, ↓ ATP, ↑ free radicals
        ↓
Further membrane damage → more vesiculation
        ↓
Phagocytosis by splenic macrophages
        ↓
EXTRAVASCULAR HEMOLYSIS

Normal RBC lifespan = 120 days. In HS = reduced to 10-20 days

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Clinical Features

Anemia + Jaundice + Splenomegaly

Severity Spectrum:

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Complications

1. Gallstones (Cholelithiasis) - Most Common Complication

• Pigment stones (bilirubinate) due to chronic ↑ unconjugated bilirubin
• Usually appear in adolescents and young adults
• Can cause biliary colic, cholecystitis, obstructive jaundice

2. Hemolytic Crises

• Triggered by viral illnesses (usually before age 6)
• Sudden worsening of jaundice + ↑ splenomegaly + ↓ hematocrit
• Generally mild and self-limiting

3. Aplastic Crises - Most Dangerous

• Caused by Parvovirus B19 (B19 infects erythroid progenitors)
• Bone marrow suppression → sudden severe drop in Hb + reticulocyte count
• Presents with: fever, vomiting, pallor, severe anemia
• May require transfusion

4. Megaloblastic Crisis

• In states of high folate demand: pregnancy, growing children, recovering from aplastic crisis
• Folate depletion → megaloblastic change on top of hemolysis

5. Uncommon Complications

• Skin ulceration, gout, chronic leg dermatitis
• Cardiomyopathy, spinal cord dysfunction, movement disorders
• Extramedullary hematopoiesis (in severe untreated cases) → hand/skull deformities

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Peripheral Blood Smear - Key Finding

• Spherocytes: Small, dense, round cells with no central pallor
• Polychromasia: ↑ reticulocytes (large bluish cells)
• MCHC elevated (>36 g/dL) - hallmark finding (dense cells)
• MCV normal or slightly low

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Laboratory Diagnosis

The EMA (eosin-5'-maleimide) binding test by flow cytometry is now the preferred diagnostic test - sensitive and specific.

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Conditions with Spherocytes on Blood Smear (Differential Diagnosis)

• Autoimmune hemolytic anemia (Coombs positive - key distinguisher)
• Thermal injury (burns)
• ABO incompatibility in newborns
• Clostridial septicemia
• Microangiopathic hemolytic anemia

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Treatment

1. Splenectomy - Definitive Treatment

• Removes the main site of RBC destruction
• Does NOT correct the membrane defect, but eliminates hemolysis in most patients
• RBC lifespan normalizes after splenectomy
• Timing: Usually deferred until age 6+ (to preserve immune function against encapsulated bacteria)
• Post-splenectomy: Vaccinate against Streptococcus pneumoniae, H. influenzae type b, Neisseria meningitidis before surgery; lifelong penicillin prophylaxis in children

2. Folic Acid Supplementation

• All patients with ongoing hemolysis should take folic acid 1 mg/day to prevent megaloblastic crisis

3. Blood Transfusions

• For severe anemia, aplastic crises, or severe neonatal HS

4. Cholecystectomy

• If gallstones develop; often done simultaneously with splenectomy

5. Partial Splenectomy

• Considered in young children where immune protection of the spleen is important

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Summary

HEREDITARY SPHEROCYTOSIS

Cause:    Mutation in RBC membrane proteins
          (Ankyrin > Band 3 > β-Spectrin > Protein 4.2 > α-Spectrin)

Genetics: 75% autosomal dominant

Result:   Membrane vesiculation → spherocytes
          → trapped in spleen → extravascular hemolysis

Triad:    ANEMIA + JAUNDICE + SPLENOMEGALY

Key lab:  ↑ MCHC, spherocytes on smear, negative Coombs

Test:     EMA binding test (flow cytometry) - best screening

Treat:    Splenectomy (curative for hemolysis) + folic acid
          + vaccinate before splenectomy