Question
Describe the five types of immunoglobulins (antibodies) — IgG, IgA, IgM, IgE, and IgD — with their structure, location, and functions.
Solution — Step by Step
All antibodies share the same basic Y-shaped structure: two identical heavy chains and two identical light chains linked by disulfide bonds.
Key regions:
- Fab region (Fragment antigen-binding): the two “arms” of the Y — variable regions that bind specifically to antigens
- Fc region (Fragment crystallizable): the “stem” of the Y — constant region that interacts with immune cells and complement proteins
- Hinge region: flexible area allowing the Fab arms to move and engage antigens at varying distances
Structure: Monomer (single Y unit)
Location: Blood, tissue fluids; accounts for ~75-80% of serum antibodies
Functions:
- Primary antibody of secondary immune response (memory response)
- Only antibody that crosses the placenta → provides passive immunity to newborns (up to 6 months)
- Activates complement system
- Promotes opsonization (coating pathogens to enhance phagocytosis)
- Neutralizes bacterial toxins and viruses
NEET focus: IgG is the “memory antibody” — its high concentration after secondary exposure is why vaccines work.
Structure: Dimer (two Y units joined by a J-chain and secretory component)
Location: Secretions — saliva, tears, breast milk, mucus of respiratory tract, GI tract, urogenital tract
Functions:
- First line of defence at mucosal surfaces
- Prevents pathogen attachment to mucosal epithelium
- Secretory IgA in breast milk protects newborns against gut infections
- Does NOT activate complement
Key fact: Colostrum (first milk) is extremely rich in IgA, providing newborns with passive mucosal immunity.
Structure: Pentamer (five Y units joined by J-chains, forming a star shape)
Location: Blood, first immunoglobulin produced in B-cell development
Functions:
- First antibody produced during primary immune response
- Very effective at complement activation (10× more efficient than IgG)
- Agglutination — its pentameric structure can cross-link many antigens simultaneously
- ABO blood group antibodies are IgM (anti-A, anti-B)
- IgM levels high → recent/current infection
NEET focus: The pentameric structure of IgM with 10 antigen-binding sites makes it the most effective agglutinating antibody.
Structure: Monomer
Location: Bound to mast cells and basophils; extremely low concentration in blood
Functions:
- Mediates allergic reactions and hypersensitivity (Type I hypersensitivity)
- When antigen (allergen) binds to IgE on mast cells → mast cells degranulate → release histamine → allergic symptoms
- Also important in defence against parasitic infections (helminths/worms)
- Elevated IgE levels indicate allergic disease or parasitic infection
Key fact: IgE is the lowest concentration antibody in serum (~0.05% of total immunoglobulins) but triggers the most dramatic responses.
Why This Works
The diversity of antibody classes reflects the diversity of threats the body faces. Mucosal surfaces need IgA. Blood-borne pathogens need IgG. Rapid response needs IgM. Parasites and allergens trigger IgE. IgD remains less well understood but appears to signal B-cell activation.
The structural differences (monomer vs dimer vs pentamer) are directly related to function. IgA’s dimer allows it to be secreted through epithelial cells. IgM’s pentamer makes it enormously efficient at complement activation and agglutination.
Alternative Comparison — Quick Table
| Class | Structure | Location | Key Function |
|---|---|---|---|
| IgG | Monomer | Blood | Memory, placental transfer |
| IgA | Dimer | Secretions | Mucosal defence |
| IgM | Pentamer | Blood | Primary response, complement |
| IgE | Monomer | Mast cells | Allergy, antiparasitic |
| IgD | Monomer | B-cell surface | B-cell activation signal |
Common Mistake
Students confuse IgM and IgG in primary vs secondary immune responses. IgM is the first antibody produced (detectable within days of infection). IgG comes later but persists as the memory antibody. In a second exposure to the same antigen, IgG rises rapidly and dramatically (the anamnestic response). So: IgM = “first time,” IgG = “stronger time.”