Allelic Exclusion
 
The clonal selection theory put forward by Burnet (1) and Jerne (2) proposes that each single lymphocyte expresses only one single type of antibody. This theory received its first structural confirmation when it was shown that patients with multiple myeloma, a proliferative disorder of plasma cells, had in their serum a highly elevated level of only one molecular species of immunoglobulin, originating from the corresponding malignant clone. Single-cell experiments performed by the Nossal group in Australia (3) indicated that each cell isolated from mouse immunized with Salmonella antigens produced only one antibody of one given specificity. These observations were in agreement with the clonal theory, but did not explain by which mechanism this was achieved. An elegant approach to that problem was made with the analysis of allotypes in heterozygous animals. Allotypes, as defined by Oudin (4), are antigenic specificities that are shared by a group of animals within a given species. These genetic markers are allelic variants of immunoglobulins that are found on both the heavy and light chains. They have been studied extensively in the rabbits, mice, and humans. In most cases, they are the result of a few amino acid substitutions that confer specific epitopes (or allotopes) that can be recognized by specific antibodies. In a rabbit heterozygous for H- and L-chain allotypes, Oudin showed that any given immunoglobulin molecule always had two identical heavy chains and two identical light chains with respect to the corresponding allotypes. This ensured that the Ig molecule was symmetrical and therefore had two identical antibody combining sites, an obvious prerequisite of the clonal theory. The next step was made at the intracellular level, when Pernis et al. (5) showed with specific anti-allotype antibodies that, in a heterozygous rabbit, any given cell always expressed only one allotype of each heavy or light chain. This suggested that a B cell, although diploid, had only one of its two alleles functional, and the phenomenon was termed “allelic exclusion.”
 The molecular mechanisms that account for this phenomenon could be understood only when the complex mosaic structures of immunoglobulin (Ig) genes were deciphered. Each Ig heavy or light chain contains an NH₂-terminal variable and a COOH-terminal constant region, each encoded by a multiplicity of gene segments. Diversity of the variable regions, which is directly linked to the  necessity to produce a very large number of different antibody molecules with a limited number of genes, is generated by specific mechanisms of gene rearrangements that take place exclusively in lymphocytes. The heavy-chain variable region is encoded after the rearrangement of three discrete sets of germline gene segments (V_H, D, J_H), and two for the light chain (V_L, J_L). Gene rearrangements take place sequentially during B-cell differentiation in the bone marrow, in a strictly regulated manner, as initially proposed by Alt and Baltimore. The heavy-chain gene segments rearrange D to JH and V_H to D–J_H. The gene-segment joinings are random, so only one-third of the rearranged genes have an open reading frame. As soon as one rearrangement is in frame, the corresponding m chain is expressed. Whenever the first rearrangement is successful, the second allele remains in the germline configuration, which indicates that a regulatory mechanism has taken place. It was shown by transfection experiments and by making m transgenic mice that the negative feedback inhibition of the gene rearrangement of the second allele was controlled by the m chain itself, when expressed at the cell surface. It was also shown that, at this “preB stage,” the m chain was associated with a surrogate light chain (YL), which is monomorphic and composed of two polypeptide chains encoded by the l5 and VpreB genes. Expression of m–YL has been shown to down-regulate the heavy-chain gene rearrangement, whereas rearrangement of the light chain locus is switched on, in the same random reading frames as above. The final result of this complex sequence of events is that, at each locus, only one allele is functional; the other one either remains in the germline configuration or has an out-of-frame rearranged gene, thereby accounting for the allelic exclusion phenomenon.
References
1.M. F. Burnet (1959) The Clonal Selection Theory of Acquired Immunity, Vanderbilt University Press, Nashville, TN. 
2. N. K. Jerne (1955) The natural selection theory of antibody formation. Proc. Natl. Acad. Sci.  USA 41, 849–857. 
3. G. J. V. Nossal, A Szenberg, G. L. Ada, and C. M. Austin (1964) Single cell studies on 19 S antibody production. J. Exp. Med. 119, 485–502. 
4. J. Oudin (1960) Allotypy of rabbit serum proteins. I. Immunochemical analysis leading to the inoliviolualization of seven main allotypes. J. Exp. Med. 112, 107–124. 
5. B. Pernis, G. Chiappino, A. S. Kelus, and P. G. H. Gell (1965) Cellular localisation of immunoglobulins with different allotypic specificities in rabbit lymphoid tissues. J. Exp. Med. 122, 853–876. 
6. F. Alt (1984) Exclusive immunoglobulin genes. Nature 312, 502–503.