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Antibody (Immunoglobulin): Classification, Structure, and Function

Definition

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Basic Structure of an Antibody

Four-Chain Unit

• Two identical heavy (H) chains (~55,000 Da for IgG/IgA/IgD; ~70,000 Da for IgM/IgE)
• Two identical light (L) chains (~24,000 Da each)
• Chains held together by inter-chain disulfide bonds and non-covalent forces

Regions of the Antibody

Variable vs. Constant Regions

• The N-terminal ~110 amino acids of both H and L chains show high sequence variability - these are the Variable (V) domains, containing three hypervariable regions (CDRs) that form the antigen-binding site.
• The C-terminal portions are the Constant (C) domains - these are conserved within each class and determine isotype and effector function.

Light Chains

Heavy Chains

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Classification of Antibodies

1. By Class (Isotype) - 5 Classes

2. By Subclass

• IgG: 4 subclasses - IgG1, IgG2, IgG3, IgG4 (differ mainly in hinge region)
• IgA: 2 subclasses - IgA1 (long hinge, susceptible to bacterial proteases), IgA2 (short hinge, protease-resistant)
• IgM: 2 subclasses described but less clinically significant

3. By Light Chain Type (Type)

• Kappa (κ) type or Lambda (λ) type - applies to all classes

4. By Form

• Monomeric: IgG, IgE, IgD, serum IgA
• Dimeric: Secretory IgA (with J chain + secretory component)
• Pentameric: IgM (with J chain)

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Structure and Function of Each Antibody Class

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IgG

• Monomeric four-chain unit (2 γ heavy chains + 2 light chains)
• Molecular weight: ~150 kDa
• Hinge region connects Fab arms to Fc (flexible, protease-sensitive)
• 3 constant domains in heavy chain (Cγ1, Cγ2, Cγ3)
• Cγ2 domains do NOT closely interact but carry N-linked carbohydrate between them
• Cγ2 domain carries binding sites for complement C1q and Fc receptors
• 4 subclasses (IgG1-4): IgG1 and IgG3 most effective at complement activation and ADCC; IgG4 can undergo Fab arm exchange to produce bispecific molecules
• Serum concentration: 8-16 mg/mL (constitutes ~80% of total serum immunoglobulin)
• Half-life: 18-23 days (longest of all isotypes, maintained by neonatal Fc receptor, FcRn)

1. Major antibody of secondary (anamnestic) immune response - produced copiously on re-exposure to antigen
2. Opsonization - Fc region binds to FcγR on macrophages and neutrophils, enhancing phagocytosis of IgG-coated microorganisms
3. Complement activation (classical pathway) via C1q binding to Cγ2 - IgG1 and IgG3 > IgG2 >> IgG4
4. Antibody-Dependent Cell-mediated Cytotoxicity (ADCC) - guides NK cells to destroy IgG-coated target cells
5. Placental transfer - the ONLY immunoglobulin that crosses the placenta (via FcRn), providing passive immunity to the neonate
6. Neutralization of toxins, viruses, and bacteria
7. Widely distributed in blood and extravascular tissue fluids (48% intravascular)

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IgM

• Secreted form is a pentamer of five four-chain units joined by a J (joining) chain (15 kDa acidic glycoprotein)
• Molecular weight: ~900 kDa; sedimentation rate: 19S
• Heavy chain: μ, with 5 constant domains (Cμ1-Cμ4 + extra domain replaces hinge region)
• 10 antigen-binding sites (though due to steric constraints, functional valency for large antigens may be lower)
• J chain links two adjacent Fc regions at carboxy-terminal tailpieces via disulfide bonds
• Monomeric IgM is expressed as B-cell receptor (BCR) on naive B lymphocytes (membrane-anchored form with transmembrane sequence)
• Serum concentration: 1.2-4.0 mg/mL (~13% of total Ig); half-life: 5-6 days

1. First antibody produced in primary immune response - first isotype expressed by developing B cells and by the body upon initial infection
2. Most efficient complement activator (classical pathway) - a single IgM molecule bound to antigen can activate C1q (requires two adjacent Fc regions; IgG requires two nearby molecules)
3. Powerful agglutinating antibody (+4 potency) due to 10 binding sites - effective at cross-linking particulate antigens, bacteria, and red blood cells
4. B-cell antigen receptor - monomeric surface IgM is the primary antigen recognition molecule on naïve B cells
5. Confined largely to blood (41% intravascular) due to large size; does NOT cross the placenta
6. Phylogenetically the most primitive immunoglobulin; μ chain gene is believed to be the evolutionary precursor of other heavy chain genes

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IgA

• Exists in three forms:
  • Serum IgA: mainly monomeric (small amounts of dimer); molecular weight ~160 kDa
  • Secretory IgA (sIgA): dimer - two monomers linked by J chain + secretory component (SC)
  • SC is derived from the poly-Ig receptor on epithelial cells; it protects IgA from proteolytic degradation in secretions
• Heavy chain: α; 3 constant domains + hinge region
• IgA1: long O-linked glycosylated hinge; susceptible to bacterial IgA1 proteases
• IgA2: very short hinge; more resistant to bacterial proteases; predominates in lower GI tract
• Serum concentration: 0.4-2.2 mg/mL (~6% of total Ig); half-life: 5-6.5 days
• Fastest catabolized of the major immunoglobulins (24%/day)

1. Primary defense of mucosal surfaces - sIgA is the dominant antibody in saliva, tears, breast milk, colostrum, intestinal secretions, respiratory and urogenital secretions
2. Immune exclusion - sIgA prevents attachment of pathogens to epithelial surfaces (first line of mucosal defense)
3. Provides passive immunity to neonates via breast milk
4. Serum IgA can link pathogens to effector cells via FcαR (Fc receptor for IgA)
5. Does NOT activate complement via classical pathway; may activate the alternative pathway
6. Present in external secretions at high levels (+4 potency)
7. IgA deficiency is the most common primary immunodeficiency (1 in 600 persons)

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IgE

• Monomeric four-chain unit
• Molecular weight: ~190 kDa (heavier than IgG due to extra domain)
• Heavy chain: ε; 4 constant domains (Cε1-Cε4) - NO hinge region (replaced by extra domain pair, similar to IgM)
• Lowest serum concentration of all Igs: 17-450 ng/mL (~0.002% of total Ig)
• Half-life in serum: very short (~2-3 days), but when bound to mast cell FcεRI receptors, half-life extends to weeks
• Most IgE in the body exists bound to high-affinity FcεRI receptors on mast cells and basophils rather than free in serum

1. Mediator of immediate (Type I) hypersensitivity reactions (allergy and anaphylaxis): Antigen (allergen) cross-links IgE bound to mast cell FcεRI → mast cell degranulation → release of histamine, leukotrienes, prostaglandins → acute inflammatory reaction (urticaria, bronchoconstriction, anaphylaxis)
2. Anti-parasitic defense - particularly against helminth (worm) infections; IgE-mediated eosinophil activation and mast cell responses are protective against large extracellular parasites
3. Fixation to homologous mast cells and basophils (+4 potency)
4. Elevated in atopic conditions: asthma, allergic rhinitis, atopic dermatitis, and parasitic infections
5. Does NOT activate complement and does not cross the placenta

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IgD

• Monomeric four-chain unit
• Molecular weight: ~185 kDa
• Heavy chain: δ; 3 constant domains (Cδ1-Cδ3) with a hinge region bearing O-linked carbohydrates and a unique polyanionic region (alternating positive/negative charges)
• Has tailpieces at the C-termini of heavy chains
• Extremely low serum concentration: ~0.03 mg/mL (~1% of total Ig)
• Shortest half-life: ~2.8 days (fastest catabolized along with IgE; ~37%/day)
• Very sensitive to proteolytic degradation

1. B-cell antigen receptor (BCR) - the primary function of IgD is as a surface receptor on mature naive B lymphocytes (co-expressed with surface IgM); it participates in B-cell activation and signaling
2. As a co-receptor with IgM on B cells, IgD may lower the threshold for B-cell activation or participate in B-cell tolerance
3. Serum function is poorly understood - it does not fix complement, does not cross the placenta, and does not bind to mast cells
4. Recent research suggests IgD may play a role in upper respiratory immunity and in amplifying immune surveillance at mucosal sites
5. Principally bound to B cells (75% intravascular)

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Summary Comparison Table

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Key Points for 15-Mark Answer

1. All antibodies share the basic four-chain Y-shaped monomer of 2H + 2L chains linked by disulfide bonds
2. Fab binds antigen; Fc mediates effector functions
3. CDRs (hypervariable regions) within V domains are the direct antigen contact points
4. Five classes (isotypes) defined by heavy chain constant regions: γ, μ, α, ε, δ
5. IgG = major serum Ig, secondary response, opsonization, ADCC, placental transfer
6. IgM = primary response, best complement activator, confined to blood, B-cell receptor
7. IgA = mucosal immunity, secretory form (sIgA) with J chain + secretory component
8. IgE = allergy/anaphylaxis (type I hypersensitivity), anti-parasitic defense
9. IgD = mainly B-cell surface receptor; function in serum unclear

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• Roitt's Essential Immunology, Chapter 3 - The structure and function of immunoglobulin classes, pp. 92-98
• Henry's Clinical Diagnosis and Management by Laboratory Methods, Chapter 47 - General Properties of Immunoglobulins, pp. 1091-1094 (Tables 47.3 & 47.4)