What's your age again? Determination of human neutrophil half-lives revisited

Neutrophils are the most abundant white blood cells and are indispensable for host defense. Recently, they have also been implicated in immune regulation and suppression. The latter functions seem hard to reconcile with the widely held view that neutrophils are very short-lived, with a circulatory half-life of < 7 h. To reopen the discussion on the average neutrophil half-life, we review and discuss experiments performed in the 1950s, 1960s, and 1970s, as well as recent in vivo labeling experiments. We reappraise the current knowledge on neutrophil half-lives, including their production in the bone marrow, their residency in the circulation and marginated pool, and their exit from the circulation.