Bio 347 Chapter 5. Major Concepts
I. Concepts for the basis of antigen recognition
1. Both immunoglobulins and the T-cell antigen receptor proteins are constructed through the use of multiple gene segments.
2. Each peptide chain is coded for by three or four gene segments.
3. Variable gene segment (V), a joining gene segment (J), and a constant gene segment (C) for light chains and a fourth gene segment called D.
4. The V,D, and J gene segments are selected at random from a large population of these segments in the genome
5. Additional sequence diversity may be generated through N-region insertion and variability at splice sites.
6. The immunoglobulin class produced by a B cell is determined by selection of one of a family of heavy-chain genes.
II. Generation of V region diversity
1. Gene rearrangement (Recombination-acting genes (RAG-1 & RAG-2)
a. light chain: 1V, 1J, 1C
b. heavy chain 1V, 1D, 1J, 1C
c. productive splicing (looping-out deletion)
d. pre B cell, kappa chain rearrangement, if non-productive, second kappa chain rearrangement, if non-productive, lambda chain rearrangement, if non-productive second lambda chain rearrangement, if non-productive then death by apoptosis.
2. Joining position--splice site is around position 96--site can vary--variability in this region of light chains
3. Base insertion: Random bases may be inserted or deleted at the V-D and D-J splice sites--called N-region addition (mediated by enzyme terminal deoxynucleotidyl transferase--adds bases to 3'ends of gene segments (1-10 bases inserted between V-D and D-J).
4. Somatic mutation
a. hypervariable regions (CR1, CR2, CR3)--CR3 is at V-J joining site
b. progressive change in the amino acid sequence as the immune response progresses--must be a result of somatic mutation.
c. mutations tightly clustered at CR1 and CR2 hypervariable regions
d. selection of "correct" mutation--cells stimulated by antigen-proliferate, those that are not die off--affinity increased (better fit)--mutations occur at a frequency --at least one mutation every time a B cell divides
III. Antibody binding specificities
Not taking somatic mutations into account, junctional site diversity in both heavy and light chains produces about 1.8 X 1016 binding specificities.- it is estimated that the immune response system may recognize 1 X 107 epitopes.
IV. Class switching
1. Selection of VDJ segments is an early event in the development of B cell
2. Selection of heavy chain C region is a late event--switches as Immune response progresses--VDJ remains separate from C region just before transcription into mRNA
3. Mechanisms of class switching: Heavy chains 5' side of each CH gene is a switch region--recombination occurs within these switch region--each switch region can recognize and splice to the switch region associated with another heavy chain gene.