The in vitro immune response in the absence of serum and its proteins: stimulating activity of zymosan.

In the absence of serum and its components, zymosan, together with 2-mercaptoethanol present in the medium, stimulated the in vitro primary immune response. Zymosan was effective at a concentration of as low as 2.5 microgram/ml. Removal of macrophages using iron powder resulted in higher recoveries of plaque-forming cells in the presence of zymosan but not of fetuin. Since insoluble zymosan may be easily removed from the culture, the test system described might be suitable for the identification of specific control factors secreted by cells during the in vitro immune response.