5 Types of Immunoglobulins or Antibodies - Structure and Function || Immunology Notes

5 Classes of immunoglobulins or Antibodies

Definition of immunoglobulin: Immunoglobulins are globular proteins  produced by plasma cells of B cells that react specifically with antigen that stimulated their production.

5 Classes of immunoglobulins Structure and Function
Which are the 5 classes of Immunoglobulins (Ig)?

IgG, IgM, IgA, IgD and IgE

1. IgG Structure and function

  • Structure: H2L2 (2 light chain and 2 heavy chain joined by di-sulphide bond)
  • Heavy Chain: unique for each class of immunoglobulin; gamma for IgG
  • Antigen binding sites: 2
  • MW: 150,000 D
  • % in serum: ~80% (most abundant antibody)
  • Most abundant antibody in blood, intestine and lymph

A summarized video on Immunoglobulins for better understanding


  • Predominant antibody in the secondary immune response
  • Promotes opsonisation or enhances phagocytosis. Phagocytes surface has receptors (ϒ-H chain ) for IgG.
  • Crosses placenta: yes (The only class of Ig that crosses placenta; provides protection to fetus and infants)
  • Fixes complement: Yes (can activate complement proteins using the Fc region; either causing lysis of pathogen by formation of membrane attacking complex(MAC) by complement proteins or enhances phagocytosis by professional phagocytes).
  • Fc Binds to: Phagocytes
  • 4 subclasses IgG1-IgG4
  • IgG2 is directed against polysaccharide antigens and forms an important host defence against encapsulated bacteria.
  • mediates Type II hypersensitivity along with IgM
  • Neutralization, Agglutination, complement activation, opsonisation (enhancing phagocytosis by professional phagocytes like macrophages, neutrophils etc) and Ab depependent cell mediated cytotoxicity (mediated by Natural killer cell)

2. IgM Structure and function

  • Structure:  5-H2L2 units + 1 J chain (joining chain) pentamer, Macroglobulin
  • Heavy Chain: µ (IgM)
  • Antigen binding sites:10 therefore  Most effective antibody
  • MW: 9,00000 D
  • % in serum: ~6%
  • Fixes complement: Yes just like IgG


  • First immunoglobulin to reach the site of infection; Predominant in primary immune response
  • Most efficient in agglutination, complement fixation, and other antibody interactions as IgM has 10 antigen-binding sites.
  • Important in defence against bacteria and viruses
  • Monomer as B cell receptor
  • Neutralization, Agglutination, complement activation,
  • Mediates Type II hypersensitivity along with IgG

3. IgA Structure and function

  • Secretory immunoglobulin
  • Structure: H2L2 units (similar to IgG) plus one molecule each of j-joining chain and secretory component
  • The secretory component is a polypeptide synthesized by epithelial cells that assist IgA passage to the mucosal surface. It also protects IgA from degradation in the intestinal tract.
  • Heavy Chain: α (IgA),
  • Antigen binding sites:4
  • MW: 385,000 D
  • % in serum: ~13% monomer


  • Neutralization, pathogen trapping in mucus that protects mucosal surface; major Ig in mucosal defence
  • Main immunoglobulin in secretions such as colostrum, saliva, tears, respiratory, intestinal and genital tract secretions.
  • Prevents attachment of microbes to the mucous membrane

4. IgD Structure and function

  • Structure: H2L2 in serum
  • Heavy Chain: delta (IgD)
  • Antigen binding sites:2
  • MW: 180,000 D
  • % in serum: <1%
  • Function: B cell receptor

5. IgE Structure and function

  • Structure: H2L2 in serum
  • Heavy Chain: epsilon (IgD)
  • Antigen binding sites:2
  • MW: 200,000 D
  • Mostly present in mucous membrane, skin and lungs
  • % in serum: <1%
  • Fc Binds to: -mast cells and basophils


  • Activation of mast cells and basophils against parasites and allergens
  • Mediates Type I anaphylactic hypersensitivity or allergic reactions

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