Overview
When an antigen enters the body, the immune system responds by producing antibodies β proteins of animal origin, called immunoglobulins, endowed with known antibody activity. Sera with high antibody levels following infection or immunization are called immune sera.
This lesson covers the basic structural framework shared by all immunoglobulins and then examines the five immunoglobulin classes β IgG, IgA, IgM, IgD, and IgE β comparing their structure, serum concentration, half-life, and unique clinical roles.
Learning Objectives
After this lesson you will be able toβ¦- Describe the basic structure of an immunoglobulin molecule
- Identify the heavy and light chains and their variable/constant regions
- Classify the five immunoglobulin classes by their heavy chain type
- Compare IgG, IgA, IgM, IgD, and IgE by concentration, half-life, and function
- Explain the clinical significance of each immunoglobulin class
Clinical Story
Why This MattersA newborn presents with signs suggestive of congenital infection. The physician orders IgM levels, knowing that IgM does not cross the placenta β so any IgM detected in the baby's blood must have been made by the fetus itself, pointing strongly toward intrauterine infection such as rubella or toxoplasmosis.
Core Concepts
Immunoglobulins are glycoproteins, each made of two pairs of polypeptide chains: two smaller light (L) chains (molecular weight ~25,000) and two larger heavy (H) chains (molecular weight ~50,000), linked by disulphide bonds.
Light chains occur in two types β kappa (ΞΊ) and lambda (Ξ») β common to all immunoglobulin classes.
The antigen-combining site is located at the amino-terminal end of the molecule, formed by both H and L chains. The carboxy-terminal half has a relatively constant amino acid sequence (constant region), while the amino-terminal half is highly variable (variable region) β it is this variability that gives antibodies their vast range of antigen specificity.
Each immunoglobulin class has a structurally and antigenically distinct heavy chain, designated by a Greek letter:
| Immunoglobulin Class | Heavy Chain |
|---|---|
| IgG | Ξ³ (gamma) |
| IgA | Ξ± (alpha) |
| IgM | ΞΌ (mu) |
| IgD | β (delta) |
| IgE | Ξ΅ (epsilon) |
IgG constitutes about 80% of total serum immunoglobulin, equally distributed between intracellular and extracellular compartments, with a half-life of about 23 days. It contains less carbohydrate than other immunoglobulins.
IgG is the only immunoglobulin transported across the placenta, providing passive immunity to newborns. It binds microorganisms to enhance phagocytosis and protects against infectious agents active in blood and tissues. Four subclasses exist: IgG1, IgG2, IgG3, IgG4.
IgA is the second most abundant class (10β13% of serum immunoglobulins), with a half-life of 6β8 days. It is the major immunoglobulin in colostrum, saliva, and tears, playing an important role in local mucosal immunity against respiratory and intestinal pathogens. IgA is resistant to digestive enzymes and inhibits microbial adherence to mucosal cell surfaces.
IgM makes up 5β8% of serum immunoglobulins, half-life about five days. It is synthesized by the fetus from 20 weeks of age and is not transported across the placenta β so its presence in a newborn indicates intrauterine infection (e.g., syphilis, rubella, HIV, toxoplasmosis). IgM is found mainly in the intravascular space and protects against blood invasion by microorganisms; deficiency is associated with septicemia.
IgD structurally resembles IgG, is intravascular, with a half-life of about three days. IgD and IgM occur on the surface of unstimulated B lymphocytes and serve as antigen recognition receptors.
IgE also resembles IgG structurally, half-life about two days, mostly extravascular and does not cross the placenta. Produced in respiratory and intestinal linings, IgE levels rise during asthma and eczema, and it is responsible for anaphylactic (type I) hypersensitivity reactions.
Laboratory Principle
Because each immunoglobulin class differs in molecular size, charge, and heavy-chain structure, laboratory techniques such as immunoelectrophoresis, nephelometry, and ELISA can separate and quantify them individually. Detecting IgM specifically, for example, allows the lab to distinguish a current/recent infection from past exposure, since IgM appears early and does not persist as long as IgG.
Reference Values
Approximate Serum Distributionβ οΈ Human sera contains IgG, IgA, IgM, IgD, and IgE in descending order of concentration. Reference ranges may vary between laboratories.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Elevated IgM in newborn | Suggests intrauterine (congenital) infection, since IgM cannot cross placenta | Investigate for TORCH infections |
| Low IgA | Common selective immunodeficiency, may coexist with IgE deficiency | Assess for recurrent mucosal/respiratory infections |
| Elevated IgE | Associated with asthma, eczema, and allergic/parasitic disease | Correlate with allergy work-up and clinical history |
Common Errors & How to Avoid Them
Cause: Confusing IgG's unique placental transport with other classes.
Prevention: Remember β only IgG crosses the placenta; IgM and IgE do not.
Cause: The Greek letters (Ξ³, Ξ±, ΞΌ, β, Ξ΅) are easy to confuse under exam pressure.
Prevention: Note that each Greek letter corresponds to the first letter matching pattern: GammaβG, AlphaβA, MuβM, DeltaβD, EpsilonβE.
Cause: Overemphasizing IgG in serological diagnosis.
Prevention: IgM rises first in a primary immune response and is key evidence of recent or ongoing infection.
Laboratory Tips from the Bench
When reading a serology report, always check whether the test measures IgM (recent infection) or IgG (past infection/immunity) β this single distinction changes clinical interpretation completely.
Colostrum-rich IgA is why exclusive breastfeeding in the first days of life is emphasized β it provides passive mucosal immunity that formula cannot replicate.
"GAMDE" in descending serum concentration: G(IgG) > A(IgA) > M(IgM) > D(IgD) > E(IgE).
Important Notes
Its large pentameric structure makes it especially effective at agglutination reactions but also confines it mostly to the intravascular space.
Both IgD and IgM sit on the surface of unstimulated (naΓ―ve) B lymphocytes, acting as antigen recognition receptors before the B-cell differentiates into an antibody-secreting plasma cell.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeAarav was born with low birth weight, hepatosplenomegaly, and a petechial rash. The pediatrician suspects a TORCH congenital infection and orders immunoglobulin studies on cord blood.
IgM does not cross the placenta, so its presence in cord blood confirms fetal synthesis β strong evidence the baby was infected in utero rather than simply receiving maternal antibodies (which would be IgG only).
- βCord blood IgM is a key screening marker for congenital infection
- βMaternal IgG crossing the placenta does not indicate active fetal infection by itself
- βImmunoglobulin class distinction is essential for accurate neonatal diagnosis
Frequently Asked Questions
IgG has the longest half-life (~23 days) of all immunoglobulin classes and is produced abundantly during the secondary immune response, which explains why it constitutes about 80% of total serum immunoglobulin.
IgA is the major immunoglobulin in colostrum. It coats the infant's mucosal surfaces, inhibiting the adherence of pathogens and providing passive local immunity in the gut before the baby's own immune system fully matures.
IgE binds tightly to mast cells and basophils. When it encounters its specific allergen, it triggers degranulation of these cells, releasing histamine and other mediators responsible for the anaphylactic (type I) hypersensitivity reaction.
Quick Revision
10-Minute ReviewKey Takeaways
- Immunoglobulins are glycoproteins built from paired heavy and light polypeptide chains
- Human sera contains IgG, IgA, IgM, IgD, and IgE in descending order of concentration
- IgG is the major immunoglobulin, transported across the placenta for passive immunity
- IgA, the second most abundant class, is present in colostrum, saliva, and tears
- IgM is synthesized by the fetus and useful in diagnosing congenital infections
- IgD and IgM serve as B-cell surface antigen receptors; IgE drives anaphylaxis
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling (NIOS). Microbiology Module β Lesson 57: Immunoglobulins.
- Roitt IM, Delves PJ. Roitt's Essential Immunology. Wiley-Blackwell.
- Kuby J. Immunology. WH Freeman & Company.