Immunoglobulin G

INTRODUCTION:
  • Molecular weight - 7 S molecule
  • General purpose antibody
  • Capacity, of producing IgG starts at 6month age 
  • Major serum Ig (80%)
  • Persists for many years after exposure
  • Normal serum concentration: 8 - 16 mg/ml
  • Half-life: 23 days
  • Least carbohydrate content
  • Only 1g that crosses placenta
  • Not synthesized by fetus
  • Predominant antibody in secondary immune response
  • Late antibody, appears after initial response by IgM
  • Highly efficient in complement fixation Et precipitation reactions
  • Small intestine (Most probably) Most abundant immunoglobin in blood
  • More effective than IgM in neutralization of toxins and viruses
  • Four subclasses: IgG1 - maximum serum concentration; IgG2 does not cross placenta
STRUCTURE:
  • IgG molecules contain two gamma heavy chains of a given subtype and two light chains (either kappa or lambda).
  • The 2 in IgG2 indicates the subclass to which the molecule belongs.
  • IgG2 contains two gamma2 chains (since a given B cell can only form one type of heavy chain).
  • The IgG molecule will contain either two kappa chains or two lambda chains, but never one of each.
  • A given cell produces immunoglobulin molecules with a single type of heavy chain.
  • IgG molecules with gamma1 chains would be of the IgG1 subclass.
PROPERTIES:
  • Major serum Ig about 85% of imunoglobulin
  • Monomeric structure
  • Can cross placenta as have receptor on placenta
  • The Fc piece of IgM & IgG fixes C1
  • Can Fixes & can activate complement (± IgG4)
  1. Opsonin antibodies and help in phagocytosis.
  2. Incomplete antibodies
  3. Best react at 37 c
SUBCLASSES:
  • IgG1 - Gamma 1 (γ1) heavy chains have 2 inter chain disulfide bond
  • IgG2 - Gamma 2 (γ2) heavy chains have 4
  • IgG3 - Gamma 3 (γ3) heavy chains have 11
  • IgG4 - Gamma 4 (γ4) heavy chains have 2
  • Ig3 have highest ability to activate complement followed by Ig1 and then Ig2 while Ig4 can't
  • Serum IgG4 is elevated at least twofold higher than 135 mg/dL in those with autoimmune pancreatitis.
CONDITIONS AFFECTING IgG:

IgG Increases in:
  • Chronic granulomatous infections
  • Infections of all types
  • Hyperimmunization
  • Liver disease
  • Malnutrition (severe)
  • Dysproteinemia
  • Disease associated with hypersensitivity granulomas, dermatologic disorders, and IgG myeloma
  • Rheumatoid arthritis
IgG decreases in:
  • Agammaglobulinemia
  • Lymphoid aplasia
  • Selective IgG, IgA deficiency
  • IgA myeloma
  • Bence Jones proteinemia
  • Chronic lymphoblastic leukemia
Exam Question
  • The serum concentration of IgG is maximum.
  • IgG1 - maximum serum concentration
  • IgG2 consist of Two gamma2 chains and two kappa chains
  • IgG crosses placenta readily and can activate complements
  • Serum IgG4 is elevated at least two fold higher than 135 mg/dL in those with autoimmune pancreatitis.
  • IgG Persists for many years after exposure
  • Rich source of IgG is Small intestine
  • Capacity, of producing IgG starts at 6month age 
  • The Fc piece of IgM & IgG fixes C1
  • Half life of IgG 23 days
Don't Forget to Solve all the previous Year Question asked on Immunoglobulin G