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.

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

     Functions:

• 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

    Function:

• 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

Function:

• 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

    Function: 

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

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