Antibody or Immunoglobulin

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Image credit: Wikimedia Commons

Antibodies, also known as immunoglobulins or Igs are secreted by plasma cells (B-lineage cells).

Immunoglobulins are Y shaped, consisting of four polypeptides (two identical heavy chains and two identical light chains). The polypeptides are bonded together by disulphide bonds.

There are variable regions at the first 110 or so amino acids of the amino-terminal region of a light or heavy chain. Variable regions fold in ways that produce grooves and bumps with certain charge distribution. Thus, only complementary antigen will able to find to them. The remainder of heavy chains make up the Fc fragment.

Light chains

The light chain has two domains. The amino (N) terminal domain is variable and the antigen binding site. The carboxy (C) terminal is the constant domain. The constant domain is either ? or ?. Both light chains will be same for an antibody.

Heavy chains

Heavy chain has a variable domain attached to several constant domains. Heavy chains determine the class of antibody. There have five classes of antibodies:

  1. Ig G (?)
  2. Ig M (?)
  3. Ig A (?)
  4. Ig E (?)
  5. Ig D (?)

Immunoglobulin G – Ig G

  • Most abundant class.
  • About 80% of the total serum immunoglobulin.
  • Two  ? heavy chains and two  or two  ? light chains.
  • Four human IgG subclasses: Ig G1, Ig G2, Ig G3 and Ig G4.
  • Biological activities:
    1. Cross placenta
    2. Complement fixation
    3. Opsonization

Immunoglobulin M – Ig M

  • 5%–10% of the total serum immunoglobulin.
  • Pentamer which contains J (joining) chain.
  • Higher valency.
  • Biological activities:
    1. Complement fixation
    2. Opsonization

Immunoglobulin A – Ig A

  • 10%–15% of the total immunoglobulin.
  • Contains J chain which allow the Ig A to bind to the secretory cells. It helps to across the epithelial lining to enter the external secretion.
  • Found in external secretions such as breast milk, saliva, tears, and mucus of the bronchial, genitourinary, and digestive tracts.


    Breast Milk Rich in Antibodies
    Image credit: Microsoft

Immunoglobulin E – Ig E


  • Low concentration in serum.
  • Mediates the immediate hypersensitivity reaction by the binding of the Fc fragments to the basophils or mast cells. It induces the degranulation and release the chemical mediators. Consequence, it triggers hypersensitivity reaction.
    allergy, hypersensitivity, immunology, biology

    Allergy (Hypersensitivity Type I)
    Image credit: Microsoft



Immunoglobulin D – Ig D

  • 0.2% of the total immunoglobulin.
  • Ig D, together with Ig M, is the major membrane bound immunoglobulin expressed by mature B cells.
  • No biological effector function has been identified.

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