| Abstract: | Activation of rat lymph-node cells in homologous serum was assessed by measuring the enhanced labelling of cells with 3H-uridine produced by PHA during a 4–6 h culture period. The degree of activation was proportional to the ratio of PHA to serum macromolecules (non-dialysable molecules) over a wide range of macromolecule concentrations. The possibility that PHA activates cells indirectly by reacting with a serum macromolecule which normally inhibits activation, was examined. No activation was produced by incubating cells in macromolecule-depleted media in the absence of PHA. Some activation was produced in macromolecule-depleted media at low PHA concentrations, but the extent of activation was only 39% of that produced in the presence of macromolecules. The results indicate that serum macromolecules both buffer cells against reaction with PHA and facilitate either the reaction of cells with PHA or the immediate cellular response to that reaction. |
