Purification of immunosuppressive factors extracted from bovine spleen (lymphoid chalone). II. Biological activity.

A highly purified extract from bovine spleen (FA) possessing immunosuppressive capacity was studied. Its tissue specificity was analysed in vivo and in vitro and the main components of FA, the well-known nucleosides deoxycytidine, deoxyinosine and thymidine, were tested in vivo. It seems that the molecule responsible for the biological activity of FA is a new substance endowed with strong specific activity and that lymphoid tissue is highly sensitive to the inhibiting capacity of this purified fraction.