Download IgG

Lec-3
Immunology
Dr. Twana A. Mustafa
Immunoglobulin
Paratope (biochemistry) is that part of the molecule of an antibody that binds to an antigen
while epitope (biochemistry) is that part of a biomolecule (such as a protein) that is the
target of an immune response.
Types of Immunoglobulin according to their structures:
Different types of Immunoglobulins
1. IgG
Structure: consists of two L chain and two H chain linked by di-sulphide bonds (H2L2).
IgG

A divalent with two identical antigen binding site.

4 subclasses based on antigenic differences in H –chain and number and location of
di-sulphide bonds namely IgG1-IgG4

IgG1 accounts 65% of total IgG

IgG2 is directed against polysaccharide antigens and forms an important host defense
against encapsulated bacteria.
Function:

Most abundant antibody in blood, intestine and lymph

Predominant antibody in the secondary immune response

The only antibody with the ability to cross placenta. Here, Fc portion binds to the cell
surface receptors of placental cells. Thus the most abundant immunoglobulin in new-borns.

IgG can activate complement (The other one is IgM)

Promotes opsonisation or enhances phagocytosis. Phagocytes surface has receptors
(ϒ-H chain ) for IgG.
2. IgA
Also called as secretory immunoglobulin
Structure:
IgA

Consists of H2L2 units (similar to IgG) plus one molecule each of j-joining chain and
secretory component. In serum, IgA exists as monomeric H2L2.

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.
Functions:

Main immunoglobulin in secretions such as saliva, tears, respiratory, intestinal and
genital tract secretions.

Prevents attachment of microbes to the mucous membrane

Does not fix complement
Why breast feeding is often insisted over bottle feeding?
A new born baby is passively immune as the baby received antibodies from the mother’s
blood before birth. During the first few weeks, baby gets majority of antibodies through
mother’s milk (colostrum) which is rich in IgA. After few weeks infants own immune
system start working and dependency to mother’s antibody stops there. Many results
suggest that bottle fed infants are more prone to many allergic diseases later as they actually
missed the IgA dose in the colostrum which contributes a lot in initiating immune response.
3. IgM
Macroimmunogobulin or also called as natural antibody
Structure: largest immunoglobulin
IgM

In serum, it is a pentamer composed of 5-H2L2 units + one molecule of J chain
(joining chain).

IgM is present as a monomer on the B cell surface where it functions as antigen
binding receptor

Pentamer has 10 antigen binding sites
Functions:

First immunoglobulin to reach the site of infection that is the major immunoglobulin
produced early in the primary 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

It functions (monomers) as antigen binding receptors on B cell surface.
4. IgD
IgD

Monomeric protein
Function:

unknown

May function as antigen receptor on B cell surface

Present in small amount in serum
5. IgE
Monomeric protein
IgE

Least abundant

Mediates anaphylactic hypersensitivity or allergic reactions

Major host defense against parasitic infection especially against Helminths (worm
infection).

Mostly present in mucous membrane, skin and lungs

Does not cross the placenta or fix complement.