The B cell receptor, or BCR, is a key component of the adaptive immune system. It's a membrane-bound antibody that recognizes specific antigens. BCR diversity is generated through somatic recombination of gene segments, leading to a vast array of antigen-binding specificities. This diversity allows B cells to effectively respond to a wide range of pathogens, contributing to immune defense. For further study, explore Kuby Immunology Edition 6th https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4839220/

1. B-Cell Receptor
& Its Diversity
___
By Ashu Goyal
INTRODUCTION
B-cells originate and mature in bone marrow. In there, they undergo various genetic
recombination and mutations that lead towards expression of a unique surface receptor on their
cell membrane known as B-cell Receptor (BCR).
B-Cell Receptor (BCR)
● The B-Cell Receptor (BCR) is a transmembrane glycoprotein present on the surface of a
B-cell.
● A B-Cell Receptor is composed of a membrane-bound immunoglobulin (mIg) molecule
and a signal transduction complex made up by two accessory proteins: Ig-α and Ig-β.

2. ● BCR controls the activation of a B-cell by enabling it to interact with the antigens
present in its vicinity.
● After the encounter of a B-cell with an antigen for which it bears a specific BCR, it
undergoes massive cell proliferation and differentiation to generate a pool of
antibody-secreting plasma B-cells and memory B-cells.
● Thus, the BCR helps a B-cell in two major functions. One is carrying out the signal
transduction cascade, and another is in the processing and presentation of degraded
antigen to a T-cell.
Components of a B-Cell Receptor
fig. 01: BCR and its components

3. A BCR is made up of two components:
1. A membrane-bound immunoglobulin molecule (mIg)
● The mIg is an immunoglobulin molecule of one isotype (IgD, IgM, IgG, IgA, or IgE).
● The mIg is identical to its secreted form with only one exception that it has a
transmembrane domain.
● The structure of mIg can be dissected in three domains:
(a) Extracellular Domain
● The extracellular domain consists of four peptide chains— two identical light (L) chains
of about 22 kDa and two identical heavy (H) chains of about 55 kDa or more.
● Each light chain is bound to a heavy chain by the means of various interactions like:
disulfide bonds, salt linkages, H-bonds, etc. to form a heterodimer (H-L).
● Same kind of interactions link two identical heavy chains (H-H).
● The domain structure study of an immunoglobulin molecule shows that both heavy and
light chains contain several homologous units.
● Each of these units are termed as domains and they form a loop of about 60 amino acids.
● Light chain contains: one variable (VL) and one constant (CL) domain. There are two
types of L-chains on the basis of the constant region sequences: Lambda (λ) and Kappa (κ).
● Heavy chain contains: one variable (VH) and three or four constant domains (CH1, CH2,
CH3 or CH4). There are five types of H-chains on the basis of the constant region sequences:
µ, α, δ, ε and γ.
(b) Transmembrane Domain
● It is a region that is absent in the secreted form of mIg.
● This region embeds the extracellular domain of mIg in the plasma membrane of B-cells.
(c) Cytoplasmic Domain/Tails
● The cytoplasmic extension of mIg is very short— usually 3 to 28 amino acids long.
● Because of this, the tails becomes unable to associate themselves with intracellular
signaling molecules (e.g., tyrosine kinases and G proteins).
● They cannot activate the signal cascade after forming an association with an antigen
and carry the message to the nucleus.

4. This poses a limitation and to overcome it, the second component of BCR comes into the play!
2. Signal Transduction Complex:
● This complex consists of two accessory proteins called Ig-α and Ig-β.
● Together, Ig-α and Ig-β form a heterodimer that is held together by a disulfide bridges.
● Both of them have a transmembrane domain that spans the plasma membrane and has a
cytoplasmic tails bearing an immunoreceptor tyrosine-based activation motif (ITAM).
● The Ig-α has a long cytoplasmic tail of about 61 amino acids and Ig-β has a tail that is 48
amino acids long.
● These tails are long enough to interact with the intracellular signaling molecules.
BCR & Antigen Interaction
A B-Cell Receptor interacts with an antigen on the basis of complementary. Each BCR bears
specific amino acid sequences in their antigen binding region that recognizes the
complementary sequences present on the antigen.
A BCR binds with an antigen at an immunologically active region called an epitope or
antigenic determinant site.
Both membrane bound and secreted form of Immunoglobulin body recognizes the same epitope
as their patent cell.
For effective binding of an antigen with a BCR, its epitope must possess following
characteristics:
1. The epitope should be a native protein made up of hydrophilic amino acids that are
are topographically accessible to the BCR
● A BCR recognizes that epitope which is present on the surface and is easily available for
the binding.
● This epitope must be made up of hydrophilic amino acids.
● Also, a BCR recognizes only a native protein that is present in its natural form.
● Any kind of mutation or degradation won't be recognized by a B-cell receptor.

5. 2. The biochemical nature of the epitope can be diverse
● A BCR possesses great diversity in its antigen binding region.
● Due to this diversity, it can bind to an epitope that can vary greatly in its biochemical
nature.
● Unlike a T-Cell Receptor (TCR), a BCR can bind with:
○ polymeric proteins
○ polysaccharides
○ lipids
○ nucleic acids
○ and etc.
3. The epitope can exist in various structural motifs
● In case of proteins, the epitope can be a primary, secondary, tertiary or quaternary
structure.
● And in the case of carbohydrates, the epitope can possess several branching chains.
After recognizing the epitope for which a B-cell bears a specific BCR, it forms a binary complex
with the antigen molecule. Unlike a TCR, B-cell receptors do not require any other factors like
MHCs to form bonds with the epitope present on the surface of an antigen.
fig.02: A B-Cell forming binary complex with an Antigen

6. Various covalent and non covalent interactions like H-bondings, sulfide bridges, salt bridges,
electrostatic interactions etc. facilitates the binding of a BCR to an antigen.
After the formation of effective interaction, BCR helps a cell in carrying out two major
functions:
(a) Initiation of signal transduction cascade
● A systemic process in which the ITAM present on the cytoplasmic tails of Ig-α and Ig-β
phosphorylates and leads towards the activation of various other signaling molecules that
carry the signals to the nucleus.
● After the nucleus receive the effective signal, it wakes up the DNA from the resting
phase and leads towards its massive replication.
● The replication produces clones of B-cells that bear the receptor with the same
specificity as their parent cells.
● The clones proliferate and finally differentiate to produce a pool of antibody-secreting
plasma cells and memory B-cells.
(b) Antigen Processing and Presentation
● BCR helps in internalisation of the antigen.
● This brings antigen in the lumen of the B-cell where various kinds of lytic enzymes
degrade the antigen into very fine fragments.
● A part of this fragment (usually a peptide and rarely lipid) is selected and brought
outside the core of the B-cell.
● On the surface of a B-cell, a cluster of immunological components called Major
Histocompatibility Complex or simply MHC is present.
● These MHC molecules bind the peptide fragment of the degraded antigen and present it
to the TCR present on a T-cell.
● This leads towards activation of the T-cell that in turn secretes a lot of chemical
components called cytokines.
● Cytokines are signaling molecules.
● They act on T-cells (autocrine signaling) and on surrounding cells like B-cells,
macrophages, dendritic cells, etc. (paracrine signaling).
● Their effect on immune cells is varied.
● They can either:
○ increase or decrease the phagocytic activity of phagocytes

7. ○ increase or decrease the antibody production
○ stimulate anti-microbial or cytotoxic properties, etc.
BCR— Diversity & Development
The diversity of BCRs expressed by an individual's B-cells is huge, and it comprises both naive
receptors that are randomly synthesised from the multiple germline genes during development,
as well as receptors that are retained after successfully binding antigen during previous
infections.
The reason behind this vast diversity lies in the fact that lymphocytes, unlike any other cells in
the body, have this ability to undergo massive somatic recombination and hypermutation at
various stages of their development.
1. The initial source of BCR diversity is the totally random recombination of V(D)J
● In humans, to build a functional VH gene, there is a random assortment in which one of
39 functional VH genes is coupled with one of 27 functional DH genes and 6 JH genes.
● And to build a functional VL gene, various functional VL and JL (λ or κ) combine
randomly with each other.
● During V(D)J recombination, DNA is cleaved by recombination enzymes RAG-1 and
RAG-2.
● The cleavage happens in such a way that it creates a hair pin structure at the cut end.
● This hair pin is a short, single-stranded DNA.
● It undergoes random cleavage by single-stranded endonucleases.
● Various repairing enzymes produce totally random nucleotide sequences by the process
called P-region nucleotide addition.
● Tdt (Terminal Deoxynucleotidyl Transferase) adds or replaces N-nucleotides particularly
on either side of the D segment within the VDJ junctions, which make up CDR-H3 in the
functional V-region.
● This is called N-region nucleotide addition.
● This phenomenon results in production of more than 107
different CD-H3 regions. It
equates to 107
different BCRs!
2. BCR diversity increases by Combinatorial Association of Heavy and Light Chains

8. ● Genome has the potential to generate about 8000 types of H-chain Gene's and 320 light
chain genes.
● Theoretically, any of the H-chain gene can combine with any of the L-chain gene to give
a function Ig molecule.
● This produces a diversity of upto 2 × 1010
Ig molecules!
3. Somatic Hypermutation gives the final blow!
● Somatic Hypermutation is a diversification effect that operates in the presence of an
antigen.
● This takes place in germinal centres of secondary lymphoid organs where a naive B-cell
bearing IgM or IgD interacts with an Ag.
● In this process, the individual nucleotides in VJ or VDJ units are replaced with
alternative nucleotides.
● It alters the specificity of encoded Ig— producing a pool of new diversity in the
BCRs!
● The rate of this mutation is one lakh times higher than spontaneous mutation.
● Although the whole mechanism has not yet been fully determined, it is believed that
somatic hypermutation increases the affinity of BCR towards an antigen.
● It makes BCR more responsive and rapid in its action.
All these processes, except for the Somatic Hypermutation, take place during the
development of a B-cell in the bone marrow.
In bone marrow, the following antigen-independent stages occur that leads towards
formation of a functional but naive B-cell:
1. Stem Cell: heavy chain (IgH) and kappa (κ) and lambda (λ) light chain (Igκ and Igλ) genes
are present in germline configuration.
In humans,
● the gene for κ chain is present on chromosome 2
● the gene for λ chain is present on chromosome 22
● the gene for H-chain is present on chromosome 14
—while in mice, the genes are located on Chromosome 6, 16, and 12 respectively.

9. fig. 03: germline configuration of λ, κ and H-chain genes in mice
fig. 04: Ig gene and protein assembly in vivo
2. Early Pro-B cell: Heavy chain gene first undergo D-J gene rearrangement with loss of
all the DNA in between. The nucleotide addition by Tdt and various other repair
enzymes leads towards addition or deletion of random nucleic acids.

10. 3. Late Pro-B cell: IgH undergoes V–DJ rearrangement. In this also, all the DNA in
between the desired combination of the genes is lost. The phenomenon of P-addition
and N-addition results in a large number of possible sequences.
4. Small Pre-B cell: At this cell stage, V-J genes of light chain combine together to produce a
function light chain. The κ chain is rearranged first. If rearrangement of κ alleles is
unsuccessful, the λ chain is rearranged.
5. Large Pre-B cell: Surface expression of heavy chain takes place. It associates itself with
a surrogate light chain to form a pre-BCR. Pre-BCR triggers allelic exclusion to prevent
rearrangement of the second allele. At the same time, pre-B cells proliferate and it leads
towards matching of different light chains with heavy chains.
The B-cell produced during above mentioned stages have not encountered any antigen yet.
The following changes in its structure occurs when it comes in contact with an antigen:
1. Immature B-cell: a VDJ segment joins first with Cμ genes to produce a complete heavy
IgM chain. But because of the absence of any regulatory switch, the Cδ gene present next to
Cμ also merges with the VDJ segment to produce IgD chain.
2. Mature naive B-cell: The IgM and IgD heavy chains combine with their respective light
chains to form a membrane bound BCR!
The B-cell thus produce undergoes various selection procedures.
The cell that survives all the metabolic tests is allowed to leave the bone marrow and circulate
in between the various lymphoid organs.
In the lymphoid organs, B-cell interacts with the antigen for which it bears the specific BCR.
● If it succeeds in finding its destined antigen, the BCR gets selected. It undergoes rapid
Somatic Hypermutation. This unique kind of genetic phenomenon produces a line of
B-cells that are equipped with a whole new BCRs.
These BCRs contain unique sequences of amino acids. Each of these BCRs are different from
others. Somatic Hypermutation can produce more than one million different BCRS. Thus,
adding to the diverse nature of the immune system.