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The major histocompatibility complex (MHC) of Gallus gallus is the B complex of which three classes of cell-membrane antigens have been clearly defined by serological, histogenetic, and biochemical methods. Two of these classes are homologous to classes I and II of mammals (B-F and B-L, respectively), while the third (B-G) is a differentiation antigen of the erythroid cell-line; the mammalian homologue of this class is still undefined. The B haplotypes comprise at least one gene of each class that displays linkage disequilibrium of a remarkable strength. The present work is the first systematic comparison by serological and histogenetic methods of the allelic products (allomorphs) of 15 haplotypes, including all of the 11 that were accepted as "standard" B haplotypes at the recent international Workshop on the chicken MHC in Innsbruck, Austria. The analysis has revealed many similarities, but only four pairs of probable identities: G2 and G12, F4 and F13, L4 and L13, L12 and L19. It appears therefore that the B-G locus is comparable in its degree of polymorphism to the class I (B-F) locus. The "standard" haplotypes are almost all of White Leghorn derivation, and preliminary typings of other breeds of chickens, and of wild chickens, indicate the existence of a much wider spectrum of allomorphs.

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The major histocompatibility complex class I heavy chain as a structural subunit of the human cell membrane insulin receptor: Implications for the range of biological functions of histocompatibility antigens ( ABSTRACTMonoclonal antibodies against some of the monomorphic determinants of major histocompatibility complex (MHC) class I molecules reduce insulin binding and precipitate '251-labeled insulin receptor preparations. A monoclonal antibody with specificity for the insulin binding site on the cell membrane insulin receptor of human cells was used to precipitate insulin receptors from human cell lines and resulted in distinct bands of Mr 1l30,000, 90,000, and 45,000. The Mr 45,000 molecules thus precipitated were subjected to

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One of the obvious ways of testing the clonal selection hypothesis is to determine the minimum number of cells needed to initiate a measurable immune response. At least in the uncompromising, and now classical, hypothesis of one antigen-sensitive cell clone per antibody specificity (1, 2) it is mandatory that a given antigenic determinant should not be recognizable by every small lymphocyte--accepting that antigen-sensitive ceils are to be found among this, in several ways, heterogenous cell population. While this extreme result, if found, would truly invalidate the hypothesis it is unfortunately not possible to define exactly the maximum frequency of antigen-sensitive cells which would still be compatible with the hypothesis. Clearly, the more complex the immunological universe is thought to be, the higher becomes the number of different clones required by the hypothesis, and the lower becomes the expected frequency of a cell belonging to one particular clone. Few immunologists would probably expect a frequency higher than 10 -4.For reasons which were suggested by earlier studies of the factor of immunization (3), the strong histocompatibility antigens were considered to provide a particularly exacting test of the clonal selection hypothesis. The graftvs.-host (GVH) reactions in chicken embryos injected intravenously with adult chicken lymphocytes were used in the present study to determine the frequency of antigen-sensitive cells with re3pect to the strong antigens of the B locus (4). It is concluded that their frequency is too high to be compatible with the orthodox version of clonal selection.