Structure of Antibody in detail

 Structure of antibody:-

Antibody Structure and Types

Antibodies, also called immunoglobulins (Ig), are glycoproteins produced by B-lymphocytes in response to antigens. They are essential components of the immune system, capable of recognizing and neutralizing foreign molecules such as bacteria, viruses, and toxins.

1. Structure of an Antibody

An antibody molecule is typically Y-shaped and consists of four polypeptide chains:

• Two Heavy Chains (H-chains): Longer chains that determine the antibody class.
• Two Light Chains (L-chains): Shorter chains that pair with heavy chains.

These chains are connected by disulfide bonds. Each chain has:

• Variable Region (V-region): Located at the tips of the Y; responsible for antigen binding.
• Constant Region (C-region): Determines antibody class and effector function.

Functional Domains:

• Fab (Fragment antigen-binding): Contains the variable region; binds antigen.
• Fc (Fragment crystallizable): Contains constant region; mediates immune effector functions like complement activation or binding to receptors on immune cells.

Diagram: Antibody

 

2. Types of Antibodies

Antibodies are classified into five main classes based on their heavy chain structure:

Antibody	Heavy Chain	Key Features	Functions
IgG	γ	Most abundant in serum; crosses placenta	Long-term immunity, opsonization, complement activation
IgA	α	Found in mucosal surfaces, saliva, tears, breast milk	Protects mucosal surfaces, prevents pathogen adherence
IgM	μ	Pentamer in serum; first antibody produced	Primary response, complement activation
IgE	ε	Low concentration; binds mast cells & basophils	Allergic reactions, defense against parasites
IgD	δ	Very low concentration; mostly on B-cell surface	B-cell activation and receptor function

 

3. Determination of Molecular Weight

The molecular weight of antibodies can be measured by:

• SDS-PAGE (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis): Separates heavy (~50 kDa) and light chains (~25 kDa).
• Gel Filtration / Size-Exclusion Chromatography: Determines the native molecular weight (~150 kDa for monomeric IgG).
• Ultracentrifugation: Measures sedimentation behavior to estimate molecular weight.

Summary:

• IgG: ~150 kDa
• IgA (monomeric): ~160 kDa; (dimeric in secretions): ~320 kDa
• IgM (pentamer): ~900 kDa
• IgE: ~190 kDa
• IgD: ~180 kDa

4. Key Points

• Antibodies recognize specific antigens via their variable regions.
• Different classes (IgG, IgA, IgM, IgE, IgD) have distinct biological roles.
• Molecular weight varies with chain composition and polymerization state.

Biotechnology MCQ Quiz

1. What is the primary function of antibodies?

Provide energy to cells Recognize and neutralize antigens Carry oxygen in blood Produce hormones

2. Which part of the antibody binds antigens?

Fab region Fc region Heavy chain constant region Light chain constant region

3. Which antibody class is most abundant in serum?

IgG IgA IgE IgM

4. Which antibody is responsible for allergic reactions?

IgG IgA IgE IgD

5. IgM exists in serum mainly as:

Pentamer Monomer Dimer Trimer

6. Which part of the antibody determines its class?

Heavy chain constant region Light chain variable region Fab region Light chain constant region

7. Which antibody is secreted in mucosal surfaces?

IgG IgA IgM IgE

8. Molecular weight of monomeric IgG is approximately:

150 kDa 50 kDa 25 kDa 900 kDa

9. Which antibody is found on the surface of immature B cells?

IgG IgA IgD IgE

10. Which technique can measure antibody molecular weight?

SDS-PAGE Gram staining PCR ELISA