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Immunoglobulin Genes

For medical students2 min readUpdated 2026-10-10

Human germline cells do not contain complete, ready-to-use antibody genes. The information for constructing the light and heavy chains of immunoglobulins is encoded as scattered DNA fragments—segments that assemble into a functional gene only during B-cell maturation.

Light λ chainsEncoded on chromosome 22

Light κ chainsEncoded on chromosome 2

Heavy H chainsEncoded on chromosome 14

DiversityAssembled from multiple V, D, J, and C segments

Organization of Genetic Material

Genes encoding immunoglobulin (Ig) molecules feature a mosaic structure. The coding regions are separated by long non-coding nucleotide sequences.

There are four main types of gene segments:

V segments (Variable) — encode the major part of the variable domain.
D segments (Diversity) — diversity segments, present only in heavy chain genes.
J segments (Joining) — joining regions.
C segments (Constant) — carry information for the constant regions and determine the antibody class (e.g., $C_\mu$ for IgM or $C_\gamma$ for IgG).

Through the random combination of these fragments, the body can synthesize millions of different antibody variants, generating a colossal diversity of protective molecules.

Synthesis of Light and Heavy Chains

The formation of a complete gene occurs in the B-cell DNA through somatic recombination.

Light chain assembly (using the $\kappa$ chain as an example):

At the DNA level, one variable (V) and one joining (J) segment join together. The intervening DNA between them is deleted.
The formed VJ complex is brought close to the constant C segment.
Transcription produces a pre-mRNA containing the VJ region, extra J segments, and the C region.
During splicing (processing), introns and unused J fragments are excised to form mature mRNA.

Heavy (H) chain assembly: The process is more complex due to the presence of D segments and requires two stages of recombination. First, a mixed exon is assembled from random D and J segments. Then, one of the $V_H$ segments joins the resulting DJ complex. The final $V_H D_H J_H$ exon is positioned next to the constant region, after which transcription begins.

Class Switch Recombination

During the primary immune response, B cells always begin by synthesizing IgM. This occurs because the $C_\mu$ gene segment is physically located at the 5' end of the coding DNA region, positioned ahead of all other constant genes.

As cells differentiate, class switching to other classes (IgG, IgA, etc.) occurs. This process is mediated by additional specific recombination. Enzyme systems delete the C segments located between the assembled variable gene and the C region of the immunoglobulin class that the new cell needs to synthesize.

Alternative Splicing: From Membrane to Secretion

Immunoglobulins can exist either as cell-membrane receptors or as free molecules in the blood. The regulation of this process depends on alternative splicing and polyadenylation of the primary transcript.

In pre-B cells (membrane-bound form): An mRNA including a specific exon is synthesized. It encodes a hydrophobic region in the C domain that acts as an "anchor," securely fixing the IgM molecule in the cell membrane.
In plasma cells (secretory form): During terminal differentiation, transcription terminates earlier—at a stop codon within an intron. Due to alternative polyadenylation, the exon containing the hydrophobic segment is excluded from the mature mRNA. As a result, shortened antibodies lacking the membrane "anchor" are produced and secreted into the bloodstream.

Mnemonic

Easy to remember chromosomes for antibody chains: kappa — 2, heavy — 14, lambda — 22. They form a sequence stepping by roughly a dozen: 2, 14, 22.

Frequently asked questions

Which enzymes (recombinases) carry out V(D)J recombination?

V(D)J recombination is initiated by the expression of the V(D)J recombinase complex genes. Sources indicate the complex includes 6 components:

RAG-1/RAG-2 recombinase (exonuclease) dimer;
DNA-dependent protein kinase;
DNA ligase IV;
Terminal deoxynucleotidyl transferase (TdT), associated with non-templated synthesis of oligodeoxynucleotides / non-templated nucleotide addition;
HMG 1/2 heterodimer;
Ku70/Ku80 heterodimer.

At the synaptic stage, RAG-1 and RAG-2 bind to the ends of the 12 and 23 spacer sequences of the gene segments to be joined, dimerize, and bring the segments close together. Double-stranded DNA cleavage is catalyzed by the RAG recombinase. HMG1/2 facilitates the approximation of the segments.

What additional mechanisms (besides V(D)J recombination) provide antibody diversity?

In addition to DNA segment recombination/combinatorics, antibody diversity is ensured by the following mechanisms:

Junctional inaccuracy: imprecise localization of DNA strand breaks during V, D, and J segment rearrangement;
Nucleotide insertions at junctions: P- and N-insertions, addition of extra nucleotides at the joints;
Utilization of 3 reading frames in the D segment, providing a threefold increase in variability;
Somatic hypermutations of V genes.

What antibody classes can be formed during isotype switching after IgM?

During isotype switching following primary IgM synthesis, other antibody classes can be formed via targeted DNA recombination influenced by cytokines. These include:

Immunoglobulins G (IgG) — subclasses IgG1, IgG2, IgG3, and IgG4 are formed.
Immunoglobulins A (IgA) — subclasses IgA1 and IgA2 are formed.
Immunoglobulins E (IgE) — synthesized, for example, under the influence of interleukin-4.

Switching occurs through the deletion of constant C segments located between the assembled variable gene and the C region of the new isotype.

Why is IgM synthesized first during the primary response?

The $C_\mu$ gene segment, which encodes the constant region of immunoglobulin M, is located at the 5' end of the DNA. It is physically positioned before all other C segments.

How does heavy chain assembly differ from light chain assembly?

The heavy chain contains additional D (diversity) segments. Therefore, recombination occurs in two stages: first, D and J fuse, and only then is the V segment added to them.

How does a B cell start secreting antibodies into the blood instead of anchoring them to the membrane?

During differentiation into a plasma cell, mRNA processing changes. Because transcription stops at an early stop codon, the exon encoding the hydrophobic "anchor" is spliced out, making the molecule soluble.

Go deeper

Alternative splicing mechanism using the calcitonin gene as an example
The role of stop codons in pre-mRNA processing
Post-translational modification and oligosaccharide attachment
Statistics and counts of V, D, J segments in the human genome