Immunoglobulin G (IgG)

IMMUNOGLOBULIN G(IgG)

  • This is the major source of immunoglobulin secreted by plasma cells (plasma cells develop from B cells on antigenic stimulation and secretes immunoglobulins) found in the spleen, lymph nodes and bone marrow.
  • IgG is 7S immunoglobulin with molecular weight of 160 KDa found highest concentration in blood.
  • It constitutes about 80 % of all immunoglobulin and about 20 % of plasma proteins.
  • They have lowest electrophoretic mobility and found in γ fraction.

Structure

  • It has two heavy γ chains and two light chains of either κ or λ types but not both. Fc region has CH1, CH2 and CH3 domains.
  • It is smallest of all immunoglobulins and can easily escape the blood vessels into the area of inflammation and distribution in extra vascular and intravascular compartments.
  • It is found in almost all tissue fluids and secretion except CSF.
  • It is the only immunoglobulin can pass placental barriers and found in newborn because of passive immunization.
  • Based on antigenic and structural differences in heavy chains the subclasses of IgG in humans are IgG1 (65-70%), IgG2 (23-28%), IgG3 (4-7 %) and IgG4 (3-4%) and numbers were given in accordance with their decreasing average serum concentration.
  • Inter chain disulphide bindings vary between two heavy chains at their hinge region. It is less in IgG1 and more in IgG2, IgG4 and IgG3 in order. In dogs and cats also IgG have been classified into IgG1, IgG2, IgG3 and IgG4. In cattle, IgG is classified into IgG1, IgG2 and IgG3.
  • In horse it is IgG1, IgG2, IgG3, IgG4, IgG5 and IgG6. In sheep, they are classified as IgG1, IgG2 and IgG3.
  • IgG contain about 3 % carbohydrate. Half-life varies with species.
  • IgG has high affinity.

Functions

  • Act as antitoxins
  • Form antiviral antibodies
  • Act as precipitins
  • Act as complement fixing antibodies.
  • Provide passive immunity in new born animals or birds
  • It is produced later to IgM but provide long lasting immunity.
  • CH3 domains of FC region bind to macrophages for biological activity.
Last modified: Friday, 23 September 2011, 10:39 AM