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Immunoglobulins - Classification, Structure & Function

(8 Mark Exam Answer - 1st Year MBBS Biochemistry)

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INTRODUCTION

Simple memory tip: "Immuno = Immune, Globulin = Protein" - they are immune proteins!

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PART 1 - CLASSIFICATION

Memory trick: "GAMED" = IgG, IgA, IgM, IgE, IgD

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PART 2 - STRUCTURE

Basic Structure (Same for ALL Immunoglobulins)

• 2 Heavy (H) chains - large chains (~440 amino acids in IgG)
• 2 Light (L) chains - small chains (~220 amino acids in IgG)
• All 4 chains are joined by disulfide bonds (-S-S-)

Parts of Each Chain

1. Variable (V) region - at the tip of the "Y" - changes from antibody to antibody - this is where antigen binds
2. Constant (C) region - at the stem/bottom of the "Y" - same in all antibodies of a given class

Important Fragments (Produced by Enzyme Digestion)

Hinge Region

• Located between Fab and Fc parts
• Gives flexibility to the molecule so it can grab antigens at different angles
• Present in IgG, IgA, IgD
• IgM and IgE do NOT have a hinge region - instead they have an extra pair of constant domains

Light Chain Types

• Kappa (κ) - more common
• Lambda (λ) - less common
• Every immunoglobulin has EITHER two kappa OR two lambda chains (never mixed)

Structure of All 5 Classes

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PART 3 - FUNCTIONS OF EACH CLASS

1. IgG - Most Abundant (75-80%)

• Main antibody in blood and tissues
• Only immunoglobulin that crosses the placenta - gives passive immunity to the newborn baby
• Activates complement system
• Binds to phagocytes (opsonization) - helps macrophages eat bacteria
• Main antibody in secondary (memory) immune response
• Has 4 subclasses: IgG1, IgG2, IgG3, IgG4

2. IgA - Guardian of Mucosal Surfaces

• Found in secretions - saliva, tears, breast milk, intestinal juice, respiratory tract
• In secretions it exists as dimeric IgA (2 monomers) linked by a J chain and covered by Secretory Component (SC) - this protects it from digestive enzymes
• Provides first line defense at mucosal surfaces
• Does NOT activate complement (classical pathway)

3. IgM - First Responder

• Pentamer (5 monomers joined by J chain) - largest immunoglobulin
• First antibody produced in primary immune response (appears first after infection)
• Most efficient at activating complement
• Natural blood group antibodies (anti-A, anti-B) are IgM
• Monomeric IgM acts as B-cell receptor (BCR) on B lymphocyte surface

4. IgE - Allergy and Antiparasitic

• Found in very low concentration in serum
• Most IgE is bound to mast cells and basophils (via Fc receptors)
• When antigen (allergen) binds to IgE on mast cells → degranulation → releases histamine → allergic reaction (hay fever, asthma, anaphylaxis)
• Also important against parasitic worm infections (helminths)

5. IgD - B-Cell Receptor

• Found in very low amounts in serum
• Main role is as a receptor on the surface of mature naive B-cells
• Helps in B-cell activation and maturation
• Its exact function is still not fully understood

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SUMMARY TABLE (Quick Revision)

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KEY POINTS TO REMEMBER

1. ALL immunoglobulins have the same basic 4-chain Y-shaped structure (2H + 2L chains)
2. They differ in their heavy chains (γ, α, μ, δ, ε)
3. IgG = most abundant, crosses placenta
4. IgA = secretory, protects mucosal surfaces
5. IgM = pentamer, first in primary response, best complement activator
6. IgE = allergy and antiparasitic
7. IgD = mainly B-cell surface receptor
8. Variable region = antigen binding | Constant region (Fc) = effector functions

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• Draw the Y-shaped IgG structure with labels - examiners love diagrams!
• The GAMED mnemonic will help you remember all 5 classes
• Focus on IgG, IgA, and IgM the most - they have the most marks in questions
• The summary table above is perfect for last-minute revision the night before your exam

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Immunoglobulins - Classification, Structure & Function

(5 Mark Exam Answer - 1st Year MBBS Biochemistry)

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INTRODUCTION

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1. CLASSIFICATION

Memory trick: "GAMED" = IgG, IgA, IgM, IgE, IgD

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2. STRUCTURE

Basic Structure - Y-Shaped Molecule

• 2 Heavy (H) chains - large (joined at the bottom of the Y)
• 2 Light (L) chains - small (on the arms of the Y)
• All chains held together by disulfide (-S-S-) bonds

Regions on Each Chain:

• Variable (V) region - at the tips of the Y - different in every antibody - binds antigen
• Constant (C) region - at the stem of the Y - same within each class - responsible for effector functions

Two Important Fragments:

• Fab (Fragment Antigen Binding) - the two arms of the Y - binds antigen
• Fc (Fragment Crystallizable) - the stem of the Y - binds complement and Fc receptors on phagocytes

Hinge Region:

• Between Fab and Fc - gives flexibility to grab antigens
• Present in IgG, IgA, IgD
• IgM and IgE have an extra pair of constant domains instead

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3. FUNCTIONS OF EACH CLASS

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KEY POINTS

1. All Ig have same basic 4-chain structure (2H + 2L chains) shaped like a Y
2. Classified by heavy chain type (γ, α, μ, δ, ε)
3. IgG = most abundant, only one crossing placenta
4. IgA = secretory immunity (mucosal)
5. IgM = first + largest (pentamer), best complement activator
6. IgE = allergy and antiparasite
7. IgD = B-cell receptor

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• Write a 2-line intro, then draw the classification table, then the structure with a small Y-diagram sketch, then functions table - this layout looks very organized and scores full marks
• Always draw the Y-shaped structure with labels in the exam - examiners give bonus impression marks for diagrams
• The summary at the end can serve as your conclusion line

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Vitamin A & Wald's Visual Cycle

(8 Mark Exam Answer - 1st Year MBBS Biochemistry)

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INTRODUCTION

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PART 1 - VITAMIN A

A. Chemistry (Forms of Vitamin A)

Memory trick: "Retinol → Retinal → Retinoic acid" (these are progressive oxidation steps)

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B. Sources of Vitamin A

• Liver, egg yolk, butter, cheese, fish liver oils (cod liver oil), full cream milk

• Dark green leafy vegetables (spinach, fenugreek)
• Orange/yellow fruits and vegetables (carrot, mango, papaya, pumpkin)

Note: β-Carotene is converted to Vitamin A in the intestinal wall. 1 molecule of β-Carotene → 2 molecules of Retinal → 2 molecules of Retinol

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C. Absorption, Transport & Storage

• Vitamin A is fat-soluble - requires dietary fat and bile salts for absorption
• Absorbed in the small intestine
• Transported in blood by Retinol Binding Protein (RBP)
• Stored in liver (as retinyl esters) - the liver is the main storage organ
• Daily Requirement: 750 mcg/day (adult)

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D. Functions of Vitamin A

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E. Deficiency of Vitamin A

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PART 2 - WALD'S VISUAL CYCLE

Background

Key Players (Remember these first!)

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Steps of the Visual Cycle

IN DARK (Rhodopsin is formed and ready):

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WHEN LIGHT FALLS (Vision occurs):

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RECOVERY (Dark adaptation - cycle repeats):

• Reduced back to All-trans-Retinol → re-isomerized to 11-cis-Retinal
• OR carried to Retinal Pigment Epithelium (RPE) cells for re-isomerization

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Simple Flow Diagram of the Visual Cycle (Draw this in exam!)

    DARK                              LIGHT
      |                                  |
  11-cis-Retinal + Opsin           Rhodopsin
         ↓                               ↓ (photon hits)
      RHODOPSIN                  All-trans-Retinal + Opsin
      (ready)                         (bleached)
         ↑                               ↓
  Re-isomerization              Transducin activated
  (in RPE cells)                        ↓
         ↑                         PDE6 activated
  All-trans-Retinal ←─────────  cGMP → 5'-GMP
                                        ↓
                               Na+ channels CLOSE
                                        ↓
                               Hyperpolarization
                                        ↓
                             Nerve signal → Brain → VISION

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Why Night Blindness Happens in Vitamin A Deficiency

• Without enough Vitamin A, there is not enough Retinal in the body
• So Rhodopsin cannot be regenerated after bleaching
• The rod cells cannot respond to dim light again
• Result: Night blindness (Nyctalopia) - the earliest sign of Vitamin A deficiency

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QUICK SUMMARY TABLE

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1. Introduction (2-3 lines) - 0.5 mark
2. Chemistry/Forms table - 1 mark
3. Sources + Absorption - 1 mark
4. Functions table - 1.5 marks
5. Deficiency table - 1 mark
6. Wald's Visual Cycle with diagram - 3 marks (this is the high-value part - always draw the cycle!)