A quantitative study of the chorioallantoic membrane reaction in the chick embryo

Abstract

When a suspension of living adult chick leucocytes is placed in contact with the chorioallantoic membrane (CAM) of a developing chick embryo, a variable number of white focal lesions appear a few days later. The number of foci is positively correlated with the number of cells in the inoculum. The phenomenon is known to be an expression of an immunological reaction of the grafted cells against the host. F. M. Burnet and his colleagues have suggested that each focus results from the immunological activity of one and only one cell. If this presumption were confirmed the CAM system would gain in usefulness, in particular for the isolation of clones of immunologically competent cells. In the present work this ‘single-hit’ hypothesis has been subjected to a specific test of its biometrical consequences. On the single-hit model the response should be proportional to the dose. Equivalently, on the logarithmic scale of measurement adopted in these experiments, the fitted dose-response line should show a slope of unity. When suitable precautions were taken to ensure thorough dispersion of donor cells, it was possible to verify the predicted relationship. The only qualification which should be attached to the conclusions concerns the possibility of co-operative action between two different cell types, one of which is very abundant relative to the other. With this proviso, it may be concluded that the individual focus observed in the CAM reaction does indeed result from the activity of a single cell.