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Humoral and Cellular Immune Response

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OVERVIEW: Adaptive vs. Innate Immunity

• Jawetz Melnick & Adelberg's Medical Microbiology, 28e, Table 8-1

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PART 1: HUMORAL IMMUNE RESPONSE

Definition

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B Cell Development and Maturation

• B cells originate and mature in the bone marrow from pluripotent hematopoietic stem cells
• Pre-B cell growth is stimulated by IL-7 (from bone marrow stromal cells)
• B cells then mature and proliferate as primed B cells under the influence of IL-4, IL-5, and IL-6
• Each B cell undergoes VDJ recombination of immunoglobulin genes, producing a unique B cell receptor (BCR) specific for a single antigen
• Naive B cells co-express IgM and IgD on their surface
• After maturation, B cells migrate to secondary lymphoid organs (lymph nodes, spleen, MALT) to await antigen encounter
• Fischer's Mastery of Surgery, 8e, "B-Cell Maturation and Function"

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Antibody Structure

• Two identical heavy chains + two identical light chains
• A variable region (antigen-binding site, made up of variable portions of both heavy and light chains)
• A constant region - the Fc fragment, which:
  • Interacts with Fc receptors on phagocytes (opsonization)
  • Activates the complement system (for certain isotypes)
• Heavy chain genes are on chromosome 14; light chain genes on chromosome 2

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Immunoglobulin Isotypes (Classes)

• Fischer's Mastery of Surgery, 8e

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B Cell Activation: Two Signals Required

• Fischer's Mastery of Surgery, 8e; Cellular and Molecular Immunology

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T-Dependent vs. T-Independent Responses

• Require CD4+ T helper cell collaboration
• Results in: class switching, affinity maturation, long-lived plasma cells, memory B cells
• Produce IgG, IgA, IgE (high-affinity)

• Activate B cells directly through BCR cross-linking (multivalent repeating epitopes)
• Rapid but simpler response
• Mostly low-affinity IgM; no memory or affinity maturation
• Cellular and Molecular Immunology, Chapter 12

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Key Processes in T-Dependent Humoral Response

1. Germinal Centre Reaction

• Somatic hypermutation of immunoglobulin variable region genes occurs
• Affinity maturation selects for B cells with highest-affinity BCRs (low-affinity cells undergo apoptosis)
• Class switching (isotype switching): B cells switch from IgM to IgG, IgA, or IgE under cytokine instruction from T follicular helper (Tfh) cells

2. Plasma Cells

• Terminally differentiated, antibody-secreting cells
• A single B cell can give rise to ~5000 plasma cells within one week, each secreting ~2000 antibody molecules per second
• Long-lived plasma cells migrate to bone marrow and continue secreting antibody for years

3. Memory B Cells

• Long-lived resting cells
• On re-exposure to the same antigen, they rapidly differentiate into plasma cells
• Basis of the secondary (anamnestic) immune response
• Cellular and Molecular Immunology, Chapter 12

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Primary vs. Secondary Antibody Response

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Effector Functions of Antibodies

1. Neutralization - antibodies block pathogen binding to host cell receptors
2. Opsonization - IgG Fc region binds Fc receptors on macrophages/neutrophils, enhancing phagocytosis
3. Complement activation - IgM and IgG activate the classical complement pathway → lysis (MAC), opsonization (C3b), inflammation (C3a/C5a)
4. Antibody-Dependent Cellular Cytotoxicity (ADCC) - IgG-coated targets are destroyed by NK cells via Fc receptor binding
5. Mucosal protection - secretory IgA prevents pathogen adherence to mucosal surfaces
6. Mast cell / basophil activation - IgE crosslinking on mast cells triggers degranulation (allergic response and anti-parasite defense)

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PART 2: CELLULAR (CELL-MEDIATED) IMMUNE RESPONSE

Definition

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T Cell Development

• T cells originate in the bone marrow but travel to the thymus to mature
• In the thymus:
  • VDJ recombination of TCR β chain, then α chain DNA
  • Positive selection: T cells that can recognize self-MHC survive
  • Negative selection: T cells that react too strongly to self-antigens (autoreactive) are deleted (central tolerance)
• Result: mature naive T cells expressing either CD4 or CD8 emerge into the periphery

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Antigen Presentation: MHC Restriction

• MHC Class I (on ALL nucleated cells): presents endogenous (intracellular) peptides (viral proteins, tumor antigens) to CD8+ T cells
• MHC Class II (on APCs: dendritic cells, macrophages, B cells): presents exogenous (extracellular) peptides to CD4+ T cells

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T Cell Activation: Two-Signal Requirement

• CD4 or CD8 acts as a co-receptor, stabilizing the interaction

• Without signal 2, T cells become anergic (tolerant), not activated

• DNA synthesis and T cell proliferation
• IL-2 secretion and upregulation of IL-2 receptors (autocrine growth loop)
• IFN-γ secretion
• Differentiation into effector T cells

• Jawetz Melnick & Adelberg's Medical Microbiology, 28e

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CD4+ T Helper Cell Subsets

• Fishman's Pulmonary Diseases, Jawetz Medical Microbiology

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CD8+ Cytotoxic T Lymphocytes (CTLs)

• CTL precursors (CD8+ T cells) recognize antigen + MHC class I on infected/abnormal cells
• Most CD8 activation is independent of costimulation (unlike CD4+ cells)
• CD4+ T helper cells "license" dendritic cells to activate CD8+ T cells (via CD40-CD40L), or provide cytokines (IL-2, IL-6) nearby
• Once activated, CTLs produce IL-2 and IFN-γ

1. Perforin-granzyme pathway:
   • CTL releases perforin (forms pores in target cell membrane)
   • Granzymes (serine proteases) enter through pores and activate caspases → apoptosis
2. Fas-FasL pathway:
   • CTL expresses FasL (CD95L), which binds Fas (CD95) on target cell → caspase-mediated apoptosis
3. Cytokine secretion: IFN-γ activates macrophages and upregulates MHC expression

• Virally infected cells (present viral peptides via MHC I)
• Tumor cells
• Intracellular bacteria
• Transplanted cells (allograft rejection)
• Roitt's Essential Immunology; Jawetz Medical Microbiology

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Macrophage Activation in Cell-Mediated Immunity

• Th1 cells secrete IFN-γ, the most potent macrophage-activating cytokine
• Activated macrophages become "classically activated" (M1 phenotype):
  • Enhanced phagocytosis and intracellular killing
  • Increased reactive oxygen/nitrogen species production
  • Enhanced antigen presentation (upregulate MHC II)
  • Secretion of pro-inflammatory cytokines (TNF-α, IL-1, IL-6)
• This is the primary mechanism for killing intracellular pathogens such as Mycobacterium tuberculosis and Leishmania

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PART 3: IMMUNOLOGICAL MEMORY

• Faster (days vs. weeks)
• Larger magnitude
• Higher affinity antibodies (IgG, IgA, IgE predominate)
• This underpins vaccine-induced protection
• Cellular and Molecular Immunology, Chapter 12

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PART 4: INTERACTION BETWEEN HUMORAL AND CELLULAR IMMUNITY

• Th2 cells drive B cell activation, class switching, and high-affinity IgE/IgG production (humoral)
• Tfh cells in germinal centres are essential for affinity maturation and long-lived plasma cell generation
• B cells act as APCs - they capture native antigen via BCR, process it, and present peptides to CD4+ T cells via MHC II (the "hapten-carrier" effect)
• CD4+ Th1 cells help activate CD8+ CTLs by licensing dendritic cells
• ADCC bridges both arms: IgG antibodies coat targets which are then killed by NK cells (innate) and CTLs
• Successful T cell immunosuppression (e.g., in transplantation) also prevents alloantibody formation by B cells, because T cell-mediated alloimmunity drives B cell alloantibody production
• Fischer's Mastery of Surgery, 8e; Jawetz Medical Microbiology, 28e

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SUMMARY TABLE: Humoral vs. Cellular Immunity

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• Cellular and Molecular Immunology, Chapter 12 - B Cell Activation and Antibody Production
• Jawetz Melnick & Adelberg's Medical Microbiology, 28e - Adaptive Immunity
• Fishman's Pulmonary Diseases and Disorders - T Helper Cell Differentiation
• Fischer's Mastery of Surgery, 8e - B-Cell Maturation and Function
• Roitt's Essential Immunology - CTL effectors
• Harrison's Principles of Internal Medicine, 22e - Humoral and Cellular Immune Responses

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Facilitated Diffusion

Definition

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Classification of Membrane Transport

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Why Is It Needed?

• Small, nonpolar molecules (O₂, CO₂, ethanol, steroids)
• Small uncharged polar molecules (water, urea)

• Large molecules (glucose, amino acids)
• Charged ions (Na⁺, K⁺, Ca²⁺, Cl⁻)
• Hydrophilic substances

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Mechanism

1. The carrier protein has a pore large enough to partially accommodate the specific molecule
2. An intrapore binding receptor (binding site) captures the molecule
3. A conformational change occurs in the carrier protein - the pore flips from facing one side of the membrane to the other
4. Because the binding force is weak, thermal motion causes the molecule to break free and be released on the opposite side
5. The carrier returns to its original conformation and is ready to transport again
6. Net movement is always down the concentration gradient - the carrier can transport in either direction, but net flux follows the gradient

• Guyton and Hall Textbook of Medical Physiology

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Two Types of Proteins Mediating Facilitated Diffusion

1. Carrier Proteins (Transporters / Permeases)

2. Ion Channel Proteins

• Harper's Illustrated Biochemistry, 32e, Table 40-4

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Transport System Classifications (by directionality)

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Key Characteristics of Facilitated Diffusion

1. Saturation Kinetics (Vmax)

• In simple diffusion: rate increases linearly and indefinitely with concentration
• In facilitated diffusion: rate increases initially but then plateaus at a maximum rate (Vmax) as all carrier proteins become occupied (saturated)
• This is exactly like enzyme kinetics (Michaelis-Menten), with a Km for the solute

2. Specificity (Stereospecificity)

• Each carrier protein is highly specific for its substrate
• Example: GLUT transporters carry D-glucose but NOT L-glucose (the mirror-image stereoisomer)
• Other monosaccharides (D-galactose, fructose) may use related but distinct carriers

3. Competition

• Structurally similar molecules compete for the same carrier binding sites
• Example: D-galactose and phlorizin competitively inhibit GLUT-mediated glucose transport
• D-galactose is itself transported; phlorizin merely occupies the site

4. Bidirectionality

• The carrier can move molecules in EITHER direction
• Net movement is always down the concentration gradient - the side with higher concentration "wins"

5. No Energy Required

• Proceeds spontaneously as long as a concentration gradient exists
• Stops at equilibrium (when concentrations on both sides are equal)
• Costanzo Physiology, 7e; Guyton & Hall Medical Physiology

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Resemblance to Enzyme Kinetics

• Harper's Illustrated Biochemistry, 32e

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Important Examples

GLUT Transporters (Glucose Transporters)

• Normally, GLUT4 resides in intracellular vesicles in the cytosol
• Insulin triggers translocation of GLUT4 to the plasma membrane
• This increases membrane density of the transporter and Vmax of glucose uptake by 10-20-fold in insulin-sensitive tissues (skeletal muscle, adipose, heart)
• This is the mechanism by which insulin lowers blood glucose after a meal
• Guyton & Hall Medical Physiology

Amino Acid Transporters

• Multiple specific carriers for neutral, acidic, and basic amino acids
• In the intestine and kidney, some amino acid transport uses Na⁺-dependent secondary active transport (not facilitated diffusion), but basolateral exit often uses facilitated diffusion

Ion Channels

• Na⁺/K⁺ channels in neurons underlie the action potential
• Ca²⁺ channels in cardiac and skeletal muscle
• CFTR (Cl⁻ channel) - defective in cystic fibrosis

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Facilitated Diffusion vs. Simple Diffusion vs. Active Transport

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Clinical Relevance

1. Diabetes mellitus - impaired GLUT4 translocation due to insulin resistance; cells fail to take up glucose despite high blood glucose
2. Hypoglycemia in fasting - brain depends almost entirely on GLUT3 (high-affinity) for basal glucose; it is not insulin-dependent
3. SGLT2 inhibitors (gliflozins) - block Na⁺-glucose cotransporters in renal tubules; used to treat T2DM by promoting glucose excretion in urine (note: SGLT is active transport, not facilitated diffusion - but basolateral glucose exit from tubular cells back into blood uses GLUT2, a facilitated transporter)
4. Cystic fibrosis - loss of CFTR (a Cl⁻ channel) impairs facilitated Cl⁻ diffusion across epithelial cells → thick mucus in lungs, pancreatic ducts
5. Pharmacokinetics - many drugs enter cells via facilitated diffusion through organic anion/cation transporters (OAT, OCT); saturation of these transporters affects drug bioavailability and drug-drug interactions - Goodman & Gilman's Pharmacological Basis of Therapeutics

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• Guyton and Hall Textbook of Medical Physiology - Facilitated Diffusion Using Membrane Carrier Proteins
• Costanzo Physiology, 7e - Facilitated Diffusion
• Harper's Illustrated Biochemistry, 32e - Transporters and Facilitated Diffusion
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Basic Mechanisms of Membrane Transport