ANTIBODY STRUCTURE AND PROPERTIES

ANTIBODY STRUCTURE AND PROPERTIES 

Immunoglobulins are glycoprotein molecules that are produced by plasma cells (B-Cells) in response to an immunogen and which function as antibodies

 

Antibody structure

 

Heavy and Light Chains: 

All Ig have a four chain structure as their basic unit.

composed of:

v  two identical light chains (23kD) and

v  two identical heavy chains (50-70kD)

v  Disulfide bonds

v  Inter-chain disulfide bonds - The 2 light chains and the 2 heavy chains are held together by inter-chain disulfide bonds and by non-covalent interactions

v   Intra-chain disulfide binds - Within each of the polypeptide chains there are also intra-chain disulfide bonds

IMMUNOGLOBULIN CLASSES

The Ig can be divided into five different classes, based on differences in the amino acid sequences in the constant region of the heavy chains (CH). All Ig within a given class will have very similar heavy chain constant regions. Heavy chain denoted by the greek letters: α, δ, ε, γ, and μ

1)     IgG - Gamma heavy chains,    

2)     IgM - Mu heavy chains

3)     IgA - Alpha heavy chains,     

4)     IgD - Delta heavy chains

5)     IgE - Epsilon heavy chains

 

STRUCTURE ND PROPERTIES OF IMMUNOGLOBULIN

 

1. Immunoglobulin G (IgG) Structure and Properties

A) 75% of serum Ig is IgG , the major Ig in serum

b). Placental transfer - IgG crosses the placenta.

Transfer is mediated by a receptor on placental cells for the Fc region of IgG. IgG2 does not cross well.

c)  Fixes complement - IgG4 does not fix complement

d)  Binding to cells - Macrophages, monocytes and some lymphocytes have Fc receptors for the Fc region of IgG. IgG2 and IgG4 do not bind to Fc receptors.

e) IgG is a good opsonin, opsonization: The antibody prepares the antigen for phagocytic cells.

 

IgM Structure and Properties

 

a)     IgM normally exists as a pentamer, but it can also exist as a monomer.

b)    In the pentameric form all heavy chains are identical and all light chains are identical. Thus, the valence is theoretically 10.

c)     IgM has an extra domain on the mu chain (CH4)

d)    it has another protein covalently bound via a S-S bond called the J chain.

e)     J chain functions in polymerization of the molecule into a pentamer.

f)     IgM is the third most common serum Ig.

g)     IgM is the first Ig to be made by the fetus and the first Ig to be made by a immature B cells when it is stimulated by antigen

h)     Because of its pentameric structure, IgM is a good complement fixing Ig. Thus, IgM antibodies are very efficient in leading to the lysis of microorganisms

i)       Because of its structure, IgM is also a good agglutinating Ig . Thus, IgM antibodies are very good in clumping microorganisms for eventual elimination from the body.

j)       IgM binds to some cells via Fc receptors.

k)    Surface IgM exists as a monomer and lacks J chain but it has an extra 20 amino acids at the C-terminus to anchor it into the membrane

 

IgA Structure and Properties

a)       Serum IgA is a monomer but IgA found in secretions is a dimer .

b)       When IgA exits as a dimer, a J chain is associated with it.

c)       When IgA is found in secretions is also has another protein associated with it called the secretory piece or T piece;

d)       IgA which is made in the plasma cell, the secretory piece is made in epithelial cells and is added to the IgA as it passes into the secretions (secretory piece helps IgA to be transported across mucosa and also protects it from degradation in the secretions.

e)        IgA is the major class of Ig in secretions - tears, saliva, colostrum, mucus. Since it is found in secretions secretory IgA is important in local (mucosal) immunity

f)         Normally IgA does not fix complement, unless aggregated.

g)        IgA can bind to some cells - APCs and some lymphocytes

 

IgD Structure and Properties

a)     IgD exists only as a monomer.

b)     IgD is found in low levels in serum; its role in serum uncertain.

c)      IgD is primarily found on B cell surfaces where it functions as a receptor for antigen. IgD on the surface of B cells has extra amino acids at C-terminal end for anchoring to the membrane.

d)     IgD does not bind complement.

IgE Structure and Properties

a)      IgE exists as a monomer and has an extra domain in the constant region.

b)     IgE is the least common serum Ig since it binds very tightly to Fc receptors on basophils and mast cells even before interacting with antigen.

c)      Involved in allergic. Binding of the allergen to the IgE on the cells results in the release of various pharmacological mediators that result in allergic symptoms.

d)     IgE also plays a role in parasitic helminth diseases. Since serum IgE levels rise in parasitic diseases, measuring IgE levels is helpful in diagnosing parasitic infections.

e)      Eosinophils have Fc receptors for IgE and binding of eosinophils to IgE-coated helminths results in killing of the parasite.

f)      IgE does not fix complement.

 

 

 

 

 


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