The Structure of Antibody

The world of antibodies is fascinating. There are billions of them in our bodies, ready to attack any foreign substance that might harm us. They are essential to our health. But how do these strange proteins that can identify, capture, and kill other proteins look in terms of their structure? Let’s find out about the structure of antibody along with its various types.

Structure of antibody

Antibodies are the key protein molecules that allow the human immune system to identify and destroy harmful foreign invaders such as bacteria and viruses.

To begin with, all antibodies are composed of four polypeptide chains, two of which are identical light chains and the other two are identical heavy chains. The light and heavy chains are bound together by disulfide bonds as well as by non-covalent interactions. 

structure of antibody

Image source- NCERT

Antigen Binding Site

The antibody molecule forms a Y shape with two identical antigen-binding regions at the tips of the Y. These tips of arms are highly variable in sequence. Therefore, antigen binding sites vary greatly among antibodies. 

Another key point is that the antigen binding site is the region through which an antibody molecule recognizes and forms weak bonds with a specific part of the antigen, known as the epitope.

Fragments of the Antibody molecule

Antibodies can be divided into two fragments which contributes to the antigen binding. These regions are known as Fab fragments. Fab stands for Fragment antigen binding. These correspond to the two identical arms of the antibody molecule, which contain the complete light chains.

The other fragment does not contribute to antigen binding. Although, it was observed to be readily crystallizable. Hence it was named as the Fc fragment, standing for Fragment crystallizable. This fragment is the part of the antibody molecule that interacts with effector molecules and cells.

Also read- Principles of Epidemiology – My Biology Dictionary

structure of antibody IgG

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Regions of Antibody Molecule

An antibody molecule is composed of two regions namely, a constant region and a variable region. The difference between the two regions lies in their amino acid sequence.

Constant regions (C) are composed of the same amino acid sequence within a particular class of antibody such as IgG, IgE.. etc. The tail of the antibody molecule is made up entirely of constant regions of heavy chains.

Whereas the amino acid sequences of the Variable region (V) vary greatly from antibody to antibody. It is these variable regions that give rise to the peculiar shape of the antigen binding site.

Hinge Region of Antibody

The Hinge region is particularly located between the chain’s CH1 and CH2 regions. It is a short stretch of amino acids on the heavy chain which provides the molecule with flexibility. This improves the antigen binding efficiency of the antibody specifically.

Domains of Antibody Molecule

The folding of constant and variable regions in an antibody molecule result in the formation of structural units known as domains. Each light chain consists of one variable domain (VL) and one constant domain (CL). Moreover, each heavy chain has one variable domain (VH) and three or four constant domains (CH1, CH2, CH3, CH4).

variable domain

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The Fab fragment consist of two variable and two constant domains, with the two variable domains making up the variable fragment (Fv), which provides the antigen specificity of the antibody with the constant domains acting as a structural framework.

Complementarity Determining Regions

Complementarity determining regions (CDRs) are hypervariable loops present in each variable domain. They are evenly distributed between four less variable framework (FR) regions. They provide a specific antigen recognition site on the surface of the antibody. Also, their hypervariability enables antibodies to recognize vast number of antigens.

To summarize, our body’s first line of defence against foreign invaders which helps identify, capture, and neutralize pathogens such as bacteria, viruses, and toxins, are nothing but complex protein molecules. Antibodies are one of the most important classes of biological molecules which have helped living organisms survive over millions of years. Thus, they are used in many areas of science, such as medicine and biotechnology, to help detect, identify, and track specific molecules or biological substances. Hope this article was useful for understanding the structure of antibody.

Thank you reading

Team MBD

Watch more- Antibodies : structures, functions and types – YouTube

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