Monocytes are known to bear receptors for fibrin and fibrinogen (11, 12); moreover, macrophages recovered from guinea pigs undergoing delayed-type hypersensitivity

The complexity of the list o f enzymes, factors, and receptors synthesized by mononuclear phagocytes suggests that they perform m a n y functions in vivo. Recent ly monoclonal antibodies (mAb)a have been described which, when compared with other hemopoetically derived cells, recognize specific antigens on h u m a n or murine phagocytes (1-4). Al though there is as yet little information about the cellular functions in which the recognized antigenic moieties might be engaged, their expression on monocytes a n d / o r macrophages suggests that they part icipate in hitherto unrecognized activities of phagocytes. One of these monocyte antigens recognized by the mAb, UC45 (2), is the topic of the present report. It will be shown that the antigen is well known, namely fibrin. There have been a number of reports linking immune responses and lymphoid cells with coagulation (5, 6). In particular, monocytes, when exposed to activated lymphocytes or their products in vitro, are able to express a procoagulant activity (7-10). Monocytes are known to bear receptors for fibrin and fibrinogen (11, 12); moreover, macrophages recovered from guinea pigs undergoing delayed-type hypersensitivity reactions were covered with fibrin (13). This present report establishes that a subpopulation of h u m a n monocytes, when exposed to plasma, are able to form on their surfaces needle-like structures of fibrin. These needles have unique antigenic properties detected by the m Ab UC45, which reacts with the a chain of fibrin as it is found on monocytes, but very poorly with the amorphous network of the s tandard fibrin clot.