5. Functions of Antibodies
• B cell Ag receptor as mAb.
• Neutralization of Ag by sAb.
• Complement activation
• Opsonization
• ADCC
• Mucosal immunity
• Neonatal immunity (by IgG & IgA)
• Immediate hypersensitivity by IgE
6. Immunoglobulin gene families
• Located on different chromosomes for
different chains:
– H-chain gene family on Ch 14.
– Kappa light chain gene family on Ch 2.
– Lambda light chain gene family on Ch 22.
• One family multiple gene segments.
• One gene segment multiple genes for
same specific region.
7. Light Chain genes
• Encoded by 3 genes:
– V (variable gene)
– J (joining gene)
– C (constant gene)
• V & J together code for variable region ( VL).
• C gene codes for the constant region ( CL).
• A complete L chain formed by splicing of V, J &
C genes.
9. Heavy Chain genes
• Encoded by 4 genes:
– Variable region coded by 3 genes:
• VH (variable)
• JH (joining)
• DH (diversity)
–Constant region (CH) encoded by one single
gene.
–H-chain C-segment has 9 genes.
12. Rearrangement at DNA level
• V,D & J joining in H-chains; V & J joining in L-
chains.
• C region genes remain separated from V-
region gene by J genes and intron.
• A primary RNA transcript is generated.
• Only Cμ & Cδ genes are transcribed in naïve B
cells.
• The other C region genes transcribed during
class switching.
14. Rearrangement at RNA level
• Takes place during the primary RNA transcript
processing.
• The C-region gene spliced with V-region genes
to generate complete H & L chain genes.
• Plays crucial role in:
– Production of membrane bound Ab.
– Production of secreted form of Ab.
– Production of different classes of Ab.
15. Heavy chain gene rearrangement
• Rearranged V region at DNA level conatins the
following sequences from 5’ end to 3’ end:
– L-exon of joined VH gene.
– VHDHJH –combined exon.
– Intron between L & VHDHJH.
– Intron 3’ to J gene.
– Remaining JH genes, if any, followed by complete
set of CH region genes.
16. Heavy chain gene rearrangement
• The rearranged H-gene is transcribed.
• Next rearrangement at RNA level takes place.
• Differential splicing of the primary RNA
transcript leads to generation of mRNA for
both μ and δ heavy chains.
• After translational, leader peptide is cleaved
to produce a fully functional chain.
17.
18. Light chain gene rearrangement
• Occurs after the rearrangement in heavy
chain.
• Similar in pattern.
• No D-genes involved.
– Variable region rearrangement in DNA.
– Splicing with C-region gene during RNA
processing.
19.
20.
21. Antibody Diversity Mechanism
• Multigene organisations of Ig chain.
• Combinatorial joining of variable region genes.
• Junctional flexibility.
• P-nucleotide addition.
• N-nucleotide addition.
• Somatic hypermutation.
• Combinatorial association of H- and L-chain.
22. Multigene organisations of Ig chain
• Large number of variable region genes:
• David Parry
Gene
segment
Number of genes
Heavy chain Kappa chain Lambda chain
V 51 40 30
D 27 0 0
J 6 5 4
23. Combinatorial joining of variable
region genes
• Random rearrangement of genes from various
segments.
• Vast variety of combinations are possible due
to this random joining.
24. Because of the diversity contributed by junctional flexibility, P- & N-
region nucleotide addition, and somatic mutation, the actual potential
exceeds these estimates by several orders of magnitude.
+
26. P-nucleotide addition
• During DNA rearrangement, a hair-pin structure is
formed.
• This hairpin is later cleaved by an endonuclease.
• A short single strand at the end of the coding sequence
is left.
• The subsequent addition of complementary
nucleotides to this strand (P-addition) by repair
enzymes generates a palindromic sequence in the
coding joint, hence called P-nucleotides.
• Variation in the position at which the hairpin is cut thus
leads to variation in the sequence of the coding joint.
27.
28. N-nucleotide addition
• Addition of new nucleotides at the free 3’ end.
• By enzyme terminal deoxynucleotide
transferase (TdT).
• Up to 15 nucleotides can be added at the
coding joints.
• Contribute to antibody diversity.
29.
30. Somatic hypermutation
• V-region genes in B-cell, on antigenic
stimulation, undergo point mutations.
• Mutations result from nucleotide substitution.
• Seen in response to T-cell dependent protein
antigens.
• Influence the process of affinity maturation.
31. Combinatorial association of H- and L-
chain
• Specificity of antibody is determined by the V-
region of both the H & L chains.
• The possible combinations of the H and L
chains are also a source of antibody diversity.
32. Because of the diversity contributed by junctional flexibility, P- & N-
region nucleotide addition, and somatic mutation, the actual potential
exceeds these estimates by several orders of magnitude.
+
33. Multiple myeloma
• Malignant disease of plasma cells.
• Normally, H & L chains are produced in equal
amounts.
• In multiple myeloma, L-chains are synthesized
much in excess than H-chains.
• Urine of these patients show increased
amounts of L-chain secretion.
34. Clinical applications
• Understanding of immunoglobulin structure
and formation has opened up a new world of
possibilities:
– Monoclonal antibodies
– Engineering mice with human immune systems
– Generating chimeric and hybrid antibodies for
clinical use
– Abzymes: antibodies with enzyme capability
35. Summary
• Antibodies and their functions.
• Immunoglobulin gene families (H & L).
• Gene rearrangement (H & L).
• Mechanisms of Antibody diversity.
• Clinical applications.
36. References
• Immunology, 5th Edition; Kuby
• Roitt’s Essential Immunology, 10th edition
• Essentials of Immunology; S.K.Gupta
• Diverse functions for DNA and RNA editing in the
immune system. Hamilton CE, Papavasiliou FN,
Rosenberg BR. RNA Biol. 2010 Mar-Apr;7(2):220-
8. Epub 2010 Mar 29.
• B cells from the bench to the clinical practice.
Moura R, Agua-Doce A, Weinmann P, Graça L,
Fonseca JE. Acta Reumatol Port. 2008 Apr-
Jun;33(2):137-54.
