Educational articles, ImmunogeneticsGeneration of Antibody Diversity

Generation of Antibody Diversity

Antibody diversity is one of the most complicated subjects in molecular genetics, yet, it is very interesting. The generation of antibody diversity is governed by the rearrangements of different antibody coding gene segments.

In this article, we are majorly focusing on an overview of how antibody diversity is generated, what are the antibody coding gene segments and how gene segments are located.

let’s start with the definition of immunology,

“The branch of the science associated with immunity is called as an immunology and the whole system is called as an immune system”. In a simple language, we can say that the immune system is the defence mechanism of our body.

Here we are discussing different gene segments involved in the formation of different immunological proteins. 

Any of the foreign particles which possibly harm our cells are characterized as antigen and the protein which helps to destroy antigens are termed as an antibody. Antibody is a specialized antigen specific protein, each antibody is specific for a single type of antigen which means that for different types of antigen, antigen-specific different antibodies are synthesized by our immune system.

Generation
Structure of antibody polypeptide chains

Structurally, antibody is a Y shaped protein molecule which contains two light chain and two heavy chain (identical). Each light chain is made up of 220 amino acid, and each heavy chain is made up of about 440-450 amino acid and, these chains are allied by disulphide bonds.

A variable region present on each heavy and light chain at N terminus end makes it antigen specific. At the C terminus end, a constant region is present on both chains which is non variable and it provides the structural support to the antibody.

A constant region constructs effector functional domain, which is responsible for interaction of that antibody with other immune components. Depending upon the functionality of antibody, it is characterized into 5 major classes: IgM, IgD, IgG, IgE and IgA.

IgM and IgD are termed as surface antibodies which are embedded into the cell membrane and every antigen is initially comes in contact with this surface antibody.

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The light chain of the antibody is of two type: kappa light chain and lambda light chain and, is determine by the constant region of a light chain, now we can understand that though the constant region is remain unchanged it plays important role in the formation of each type of light chain.

The arrangement is hard to determine but it is easy to understand. Different DNA sequences for antibody (protein) light chain is located on different chromosomes. Rather to say gene it is called as gene sequences or gene segments because different sequences within the gene are allotted for the formation of different polypeptide chains of the variable region.

Different heavy chain, light chain, constant region and lambda and kappa chain polypeptides are synthesized by different gene segments. Now firstly we will discuss the arrangements of different gene segments for each chain.

Depending upon the function and location, two types of antibodies are present: The membrane bounded antibodies, present on the membrane of the cell and secreted antibodies, secreted from the B lymphocyte cell, induced by the antigen attack.

Arrangments of gene segments for Generation of antibody diversity:

Light chain

Light chain is of two types kappa light chain or lambda light chain.

Lets denote variable region as V,

A constant region as C and

Joining segments as J (joining segments are just a non coding intervening sequences).

Keep in mind one point that each chain (whether it is a light or a heavy chain) is made up of a long chain of polypeptides with N terminus and C terminus, it starts with N terminus at  variable region and ends at C- terminus towards the constant region. See the figure to understand it properly.

Number of gene segments for the different type of polypeptide chains:

Kappa gene segmentsLambda gene segments Heavy chain
Variable region Vk=38Vλ 33VH=38-46
Joining region Jk=5Jλ =5JH=6
Diversity region 0

DH=23
Constant region CK=1Cλ =4CH=9

Kappa light chain:

At the N terminus end the Vk gene segment is code for 95 amino acid, Thee Jk gene segments code for 13 amino acids, and at C terminus end, the Ck gene segments code for remaining amino acid of a light chain.

One chain of hydrophobic leader sequences is also present at N terminus end of kappa light chain which helps in transportation of antibody at the site of action. It is made up of 17-20 amino acids.

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Generation
kappa chain synthesis.

During the development of B lymphocyte cells recombination of the different Lk-Vk segment and Vk-Jk segments constructs the different type of antigen specific antibody. The entire segment of Lk-(Vk)Jk-non coding sequence-Ck is transcribed and the non coding sequences and other non functional sequences are removed from the mature kappa chain. Only Vk-Jk-Ck sequences remain intact for the development of a particular antibody.

Lambda light chain

This gene segments are assembled during B lymphocyte maturation as well and creates different antibody though recombination of different gene segments. The maturation process is same as kappa chain gene segments. Here the segments are denoted as Jλ, Vλ, Cλ.

Generation
lambda chain gene segments.

Heavy chain

The genetic information for coding heavy chain gene segments are arranged on LH-VH, JH and CH gene segments. Here kappa or lambda variable region is remain the same as a light chain, nevertheless, one additional gene segment is present in heavy chain segment, a “diversity segment” or “D segment”.

Generation
mature heavy chain gene segment.

D segment awards additional diversity to the antibody which is present in the variable region of a heavy chain.

The D segment is made up of the 2-13 amino acid long polypeptide chain. Here 4 separate CH gene segments are present for each Ig class heavy chain.

In humans, total 9 to 10 functional CH gene segments are present. The list of different segments are given into the table. In CH gene segments of the heavy chain, the intervening sequences are typically psuedogenes ( sequences which are similar to the functional gene but are non-function), during maturation of B lymphocyte pseudogenes are removed

Depending upon the function and location, two types of antibodies are present: The membrane bounded antibodies, present on the membrane of the cell and secreted antibodies, secreted from the B lymphocyte cell, induced by the antigen attack.

Now the membrane bounded antibodies are fixed on cell membrane and embedded into lipid layer of membrane. The C terminus end of a heavy chain antibody is hydrophobic in nature which helps them to fix in the lipid layer, whereas the C terminus end of circulating antibodies are have the hydrophilic end which makes them circulate freely.

IgM and IgD are fixed antibodies present on the surface of the cell, others are freely circulating in the circulatory system and depending upon the type of antigen, the specific antibody is secreted.

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Generation of antibody diversity by class switching:

IgM and IgD are called as primary antibodies because both are present on the surface of the cell membrane, and are firstly interact with the antigen. The secondary antibodies IgE, IgA and IgG are secreted by B cells depending upon the antigen present on the cell membrane hence this class of antibodies are termed as secondary antibodies.

Depending upon the type of antigen, instead of making one class of antibody the mature B cell starts making another class of antibody, this phenomenon is called as an antibody class switching.

Once the primary class of antibody recognized the antigen, the B cell differentiation initiated depending upon the type of antigen present on the cell membrane. Even though it synthesize some specific antibody, here it synthesizes antigen-specific antibody. 

The Constant segment of antibody decides which class of antibody will produce during the immune responds, here as the primary antibody is IgM, the CH gene segment is preliminarily same for all antibodies.

In class switching instead of producing IgM antibody, some mature B lymphocytes produces another class of antibody depending upon the gene segment arrangement of CH.

This antigen induced secondary antibody production is called class switching.  The mechanism is governed by rearrangement of different DNA segments of V, D, J and C regions followed by class switching recombination.

Final words

The mechanism of class switching is as important as gene segment re-arrangements in the generation of antibody diversity. In the next article, we will discuss the VDJ  joining, junctional flexibility, nucleotide addition and somatic hypermutation.

Till then stay connected with us.

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Story created by – Tushar Chauhan

written and reviewed by- Tushar Chauhan and Binal Tailor

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