Contact between good friends: what limiting dilution analysis taught us.
The demonstration in the 1960s that T cells collaborated with B cells in enabling antibody responses to simple protein antigens opened up the challenge of how to investigate the interactions of two rare antigen-specific lymphocytes within a vast population. One idea was that T cells made soluble factors that could activate B cells at a distance; the other was that rare T cells and B cells came into contact. Using limiting dilution analysis, we asked the question 'How many B cells could a single T cell collaborate with in the short term?' Unequivocally, the answer was just one. This implied a need for cell contact, and coupled with the observation for genetic restriction in T-cell/B-cell co-operation, seemed to make a watertight case for direct coupling. Claims of diffusible antigen-specific factors undermined the importance of our findings at that time. Remarkably, those claims have not yet been retracted and our original findings that collaborating T cells and B cells must come into contact have never achieved the recognition that they deserved.