1.Combination of gene segments results in a huge number of various variable regions of the heavy and light chains expressed by different B-cells SOMATIC GENE REARRANGEMENT 2. How B cells express one light chain species and one heavy chain species even though every B cell possesses a maternal and paternal locus of both genes. Since all other genes known at the time appeared to be expressed co- dominantly, how could B cells shut down the genes on one of their chromosomes? ALLELIC EXCLUSION THE RESULT OF SOMATIC GENE REARRANGEMENTS

Evidence for allelic exclusion Allotypes can be identified by staining B cell surface Ig with antibodies a/a b/b a/b Y B b Y B a Y B b Y Y B ab Y B a AND ALLOTYPE- a polymorphism in the Heavy chain C region of Ig Suppression of H chain rearrangement by pre-B cell receptor prevents expression of two specificities of antibody per cell

Allelic exclusion is needed for efficient clonal selection All daughter cells must express the same Ig specificity otherwise the efficiency of the response would be compromised Suppression of H chain gene rearrangement helps to prevent the emergence of new daughter specificities during proliferation after clonal selection S. typhi Antibody S. typhi

Y Y YY Suppression of H chain gene rearrangement ensures only one specificty of Ab expressed per cell. Allelic exclusion prevents unwanted responses B Self antigen expressed by e.g. brain cells S. aureus Y Y Y Y Y B Y Y Y Y Y Y Y Anti S. aureus Antibodies Y Y Y Y Y Y Anti brain Abs One Ag receptor per cellIF there were two Ag receptors per cell Y Y Y Y Y Y Y Anti S. aureus Antibodies Prevents induction of unwanted responses by pathogens

ALLELIC EXCLUSION Ig alleles are not differentially marked Close (methylated) chromatin structure Asynchronous  -locus replication Blastocyst Pre-B cell Early replicating allele is chosen to undergo demethylation and chromatin changes Nucleosomes Modified histones The early replicating allele becomes accessible to rearrangement

1.Somatic rearrangement of Ig gene segments in a highly controlled manner 2.Single B-cells become committed to the synthesis of one unique H-chain and one unique L-chain variable domain, which determine their specificities 3.In one individual a huge B-cell repertoire is generated consisting of B-cell clones with different H- and L-chain variable domains 4.This potential B-cell repertoire is able to recognize a wide array of various antigens RESULT OF SOMATIC GENE REARRANGEMENT AND ALLELIC EXCLUSION INDEPENDENT ON ANTIGEN OCCURS IN THE BONE MARROW

1.Combination of gene segments results in a huge number of various variable regions of the heavy and light chains expressed by different B-cells SOMATIC GENE REARRANGEMENT 2. Successful somatic rearrangement in one chromosome inhibits gene rearrangement in the other chromosome ALLELIC EXCLUSION 3. One B-cell produces only one type of heavy and one type of light chain COMMITMENT TO ONE TYPE OF ANTIGEN BINDING SITE 4. The B-cell pool consist of B-cells with differently rearranged immunoglobulin genes THE RESULT OF SOMATIC GENE REARRANGEMENTS INDEPENDENT OF ANTIGEN OCCURS DURING B-CELL DEVELOPMENT IN THE BONE MARROW

Allelic exclusion helps diagnose and monitor lymphoma: Due to clonal expansion of a single cell that contains a unique rearrangement the amount of cancer cells in blood or in bone marrow can be determined Can be used to monitor residual tumor cells upon treatment

Allelic exclusion is needed to prevent holes in the repertoire Exclusion of anti-brain B cells i.e. self tolerance Y Y B B One specificity of Ag receptor per cell S. aureus Anti-brain Ig AND anti-S. Aureus Ig Y Y Y B B IF there were two specificities of Ag receptor per cell Anti-brain Ig B B Deletion Anergy OR anti S.Aureus B cells will be excluded leaving a “hole in the repertoire” BUT Y Y Y B B