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For more than ten years cell fusion techniques have been applied in studies on various lymphocyte functions. Ig expression was first studied in hybrids obtained by fusing myeloma cells with fibroblasts (1) or lymphomas (2), both of which do not produce Ig, and with Ig­ producing myelomas (3) or human blood lymphocytes (4). Kohler and Milstein (5) fused a myeloma with spleen cells from immunized mice. Up to 10% of the hybrids obtained secreted antibodies specific for the immunizing antigen. This suggested that plasma cells preferenti­ ally fused with the myeloma cells, a finding which was of enormous practical value. It was found that both Band T lymphocytes could be fused with the T cell tumor BW5147, which is however not permissive for Ig synthesis (6). A very large number of T cell hybridomas were generated by fusing BW5147 with cell populations containing in vivo or in vitro activated cells (7). The hybrids showed no specific T cell functions and binding assays for T cell receptorswere not available. In particular, no hybrids were obtained which expreS1ed specific cytolytic activity that could be tested in short-term Cr­ release assays (8). However, the frustrations expressed about these failures, published in January, 1978 (9), were relieved by Taniguchi and Miller's publication a few months later of T cell hybridomas producing antigen-specific suppressor factors (10). Unfortunately, their hybrids rapidly lost factor production.

Several discoveries are noteworthy for allowing us to probe the recesses of the virus­ infected cell and to search for cryptic viral genomes which might provide clues in our studies of cancer etiology or developmental biology. One of the most notable was the dis­ covery of reverse transcriptase. This marked a momentous occasion in the history of molecular biology. Not only did it provide insight into the mechanism of persistence of retroviruses but it also provided us with an enzyme that could synthesize a DNA copy of any RNA. This DNA copy could then be used as a hybridization reagent to search for both complementary DNA and viral-specific RNA. Thus one could follow the course of any viral infection or probe in tumor cells for hidden viral genomes. Second, a great deal of credit must be given to the geneticists who isolated the various deletion mutants in the'avian retrovirus system and thus provided us with the frrst means of isolating gene-spe­ cific probes. Finally, the laboratories which have mapped the genome have provided us with the framework in which to ask very specific questions with our gene-specific probes. Recently, numerous excellent reviews concerning various aspects of the retroviruses have appeared. In this review I shall not even attempt to present a comprehensive review of retroviruses.

The cells of the immune system generate a large variety of binding sites which differ in their binding specificities and can therefore react specifically with a large variety of ligands. These binding sites are part of receptor molecules, enabling the system to react to the universe of antigens. The classical antigen receptor is the antibody molecule, and accord­ ingly the first session of this colloquium deals with a classical sub­ ject, namely antibody structure. Dramatic recent advances in this field make it possible to interrelate primary and three-dimensional struc­ ture both to each other and to function, i.e. the binding of antigen and possible reactions occurring in the antibody molecule upon antigen binding. The latter point is of particular interest since it may be relevant not only for effector functions of antibodies such as the binding of complement, but also for the triggering of a lymphocyte through its antibody receptor for antigen.

Sponsored by the National Cancer Institute (NIH)