The Behaviour of Infused Human Platelets During the First Twenty‐Four Hours After Infusion

W hen freshly collected human platelets are transfused into normal recipients, an appreciable proportion disappears extremely rapidly from the circulation and does not reappear. This proportion increases with the number of manipulations performed in the preparation of the platelet concentrate (Gardner, Howell and Hirsch, 1954), and with the period of storage before infusion (Baldini, Costea and Dameshek, 1960). Those platelets which remain in the circulation after 24 hours—so‐called ‘viable’platelets—disappear slowly from the blood in the course of a further 7–10 days (see reviews by Najean and Bernard, 1960, and Odell and Kniseley, 1962). This period of platelet survival in the circulation is similar to that determined by labelling platelets in vivo with di‐isopropylfluorophosphonate‐ 32 P (DF 32 P) (Leeksma and Cohen, 1956; Zucker, Ley and Mayer, 1961). Following the infusion of labelled platelets into normal recipients, it has also been shown that the platelet‐bound radioactivity in the blood increases after 30 minutes to reach a maximum within 24–48 hours of infusion (Adelson, Rheingold and Crosby, 1957; Reisner, Keating, Friesen and Loeffler, 1958; Aas and Gardner, 1958; and others). This is contrary to what might be anticipated. A delayed increase in the number of circulating platelets has also been shown to occur when unlabelled platelets are given to thrombocytopenic subjects (Aas and Gardner, 1958). No such change is observed, however, when DF 32 P is injected to label circulating platelets (Zucker et al., 1961); this suggests that the phenomenon is due to changes which occur during the removal of platelets from the body and/or their manipulation in vitro. The fate of those infused platelets which are immediately and irreversibly cleared from the circulation is unknown; nor is it known where the potentially viable platelets reside before their return to the circulation, nor where they are finally destroyed. Surface‐counting procedures using platelets labelled in vitro with 51 Cr have led some workers to suggest that temporary accumulation occurs in the liver, the lungs and possibly the spleen, and that the liver and spleen play some role in their final destruction (Aas and Gardner, 1958; Najean and Bernard, 1960; Najean, Larrieu and Bernard, 1961). This study is concerned particularly with the sequence of events in the brief period immediately following the infusion of 51 Cr‐labelled human platelets—a period in which some of them disappear rapidly from the circulation, and later return to it. We have carried out 36 studies of these events.