THE humoral immune response to most antigenic determinants is characterised by a highly heterogeneous population of antibody molecules derived from a large array of precursor cell clonotypes1-3. The response to certain antigenic determinants, however, is relatively restricted and may be dominated by a single homogeneous antibody species. In inbred strains of mice, such dominant antibody specificities are present in all individuals of a strain 4-9, are characterised by identical idiotypic determinants 4-8, and are linked to heavy chain allotype markers7-10. Thus, these clonotypes are presumed to be a direct reflection of genetic information present in the germ line. A good example of an antibody specificity considered to be 'germ line' is the major clonotype responsive to phosphorylcholine in BALB/c mice, which is identical in its combining region to the TEPC 15 and SI07 plasmacytoma proteins4-5. Representatives of the cell clone producing this phosphoryilcholme-specific antibody have been found in high frequency (19±6 per 106 bone marrow-derived antibody-forming cell precursors (B cells)) in both conventionally reared and germ-free BALB/c mice11.
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