2. 2
•Immunoglobulin Structure
•
An antibody or immunoglobulin (Ig) is is a large, Y-shaped a
glycoprotein produced mainly by plasma cells.
There are five classes of human antibodies; all share a basic structure
that varies in order to accomplish specific biological functions.
3. Immunoglobulin (Antibody) Structure
Enzymatic fragmentation and chemical reduction with Pepsin and papain
show that All immunoglobulin molecules have a basic structure composed of
four polypeptide chains :
1-two identical heavy chains (H) and
2-two identical light chains (L).
Each light-chain polypeptide usually consists of about 200-220 amino acids
and has a mass of approximately 25,000 Da.
Each heavy chain consists of about 440-450 amino acids and has a mass of
about 50,000 to 70,000 Da.
3
5. The two heavy chains (H) are connected together by disulfide
bonds, Also Heavy and light chains are connected to each other
by disulfide bonds. The heavy chains are structurally distinct for
each immunoglobulin class or subclass.
7. Both light (L) and heavy (H) chains contain two different
regions :
1-The constant (C)regions (CL and CH): have amino acid
sequences that do not vary significantly between antibodies of the
same class .
2-The variable (V) regions (VLand VH): have different amino
acid sequences, and these regions fold together to form the
antigen-binding sites.
8. 8
Heavy Chains
• The heavy chain also has a variable region (VH) at its amino terminal
domain. The other domains of the heavy chains are termed constant (C)
domains.
• The constant domains of the heavy chain form the constant (CH) region.
The amino acid sequence of this region determines the classes of heavy
chains.
• In humans, there are five different classes or isotypes of heavy chains
Each class of Heavy chain has a characteristic amino acid sequence that
distinguishes it from the other four classes but all five classes have
Significant percentages of amino acid sequence similarities designated
by lowercase Greek letters: Gamma (γ), Alpha (α ,) Mu (μ), Delta (δ),
and Epsilon (ε), and usually written as (G, A ,M, D, or E).
9. 9
The properties of these heavy chains determine, respectively, the five
immunoglobulin (Ig) classes: IgG, IgA, IgM ,IgD, and IgE. Each
immunoglobulin class differs in its general properties, half-life,
distribution in the body, and interaction with other components of the
host’s defensive systems.
10. 10
• In many species, there are more subclasses of some heavy chains
that differ from one another by only a few amino acids , for
example: Humans have 4 subclasses of the IgG isotype called IgG1,
IgG2, IgG3, and IgG4. IgA has two subclasses IgA1 and IgA2.
