Serum collected from BALB/c mice 6 h after the intraperitoneal injection of Freund's complete adjuvant when mixed with a soluble antigen such as bovine serum albumin or human fibrinogen forms a cytophilic Ig which is taken up by the normal mouse spleen cells. When the serum was absorbed with an aggregated form of antigen, subsequent addition of soluble antigen failed to induce formation of cytophilic Ig, but an eluate recovered from the aggregated antigen did.This property of the serum is most likely non-specific since any antigen thus far tested is effective in inducing formation of cytophilic Ig and furthermore, absorption with one kind of antigen abolished the ability of the serum to form cytophilic Ig upon subsequent addition of any other antigen. Upon fractionation of the serum on Sephadex G-200 the activity was recovered in the 7-S fraction. However, the 4-S fraction (which lacks Ig) contained a factor which in the presence of antigen and normal mouse serum or its 7-S fraction formed a cytophilic Ig. The existing evidence indicated that the cytophilic Ig was taken up by 25 to 35% of the T cells (θ positive). Furthermore, about 15% of Ig carrying cells also took up the cytophilic Ig.The cytophilic Ig belongs to the IgG (IgG2a) class and presumptive evidence suggested that it may represent a complex of Ig and antigen.
