1. Immunoglobulins (2 of 2) Ali Al Khader, MD Faculty of Medicine
Al-Balqa’ Applied University

2. Save this image in your brain
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3. Generation of antibody diversity
Germline heavy and light chain gene segments
VDJ or VJ
-Recombination
-Transcription
-Translation
Heavy and light chain polypeptides
Immunoglobulin assembly
Modified from immunology for medical students 2nd edition
Gene segments will be rearranged during B cell development to assemble a functional gene of either a light or heavy chain

4. Gene segments L leader V D diversity J joining C The variable region
4 subtypes of lambda due to 4 different constant region versions
For heavy chain
Constant region
Each gene segment is composed of a set or group of versions of that segment, e.g., Jk (joining kappa) gene set has 5 different versions of Jk
See Kuby immunology for references

5. Synthesis of light chain
After removal of introns (splicing) from primary RNA transcript
Leader peptide is removed by proteolytic enzymes after translation
modified

6. Synthesis of heavy chain
All these steps are before transcription L L
Then: transcription and splicing of the RNA primary transcript…and the leader peptide will be removed after translation, along with glycosylation L
What will happen in splicing? L L L
Visit for references..modified

7. The recombination is somatic (not germline), but when does it occur ???

8. Factors of antibody diversity
Germline diversity…due to the different versions of each segment
…51 VH,27 DH, 6 JH , 40 V kappa, 5 J Kappa, 30 V lambda and 4 J lambda
Combinatorial diversity
If we want to synthesize an Ig with kappa light chain for example

9. Factors of antibody diversity, cont’d
Junctional diversity:
…In the recombination process, we delete some nucleotides and add
others (e.g., the random addition by TdT enzyme) at junction sites
between the adjoined segments
Somatic hypermutation…see next slide

10. Somatic hypermutation
= affinity maturation
Due to: very high-rate point mutations in the V regions of heavy and light chains
…will produce antibodies with higher affinity for the antigen
…B cells expressing these antibodies are preferentially selected for becoming
plasma cells

11. Allelic exclusion
There is genetic polymorphism in the genes of VH, CH, VL, and CL
…which means that there’s difference in each heterozygous individual between the allele from the mother and that from the father
…in heterozygotes for these allotypes, each B cell either express genes from the mother or from the father (not from both)…= allelic exclusion
*but the population of B cells in the same person is mixed

12. Heavy chain isotype switching (Ig class switching)
When memory B cells are restimulated, they undergo further DNA rearrangement to juxtapose their already-rearranged VDJ gene regions to different heavy chain C region genes
Variable regions are not changed here
…so: the specificity for the antigen is the same

13. Heavy chain isotype switching (Ig class switching)
Variable region is the same
Visit for references...modified

14. This sequence is important
Lippincott’s illustrated reviews immunology 2nd edition..modified

15. How does the unstimulated B cell express both IgM and IgD?

16. Important enzymes V(D)J recombinases
-For cleaving and rejoining DNA segments
-RAG-1 and RAG only found in lymphocytes (if absent: problem in lymphocyte development)
= recombination activating gene
Terminal deoxynucleotidyl transferase (TdT)
…adds nucleotides

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