THE part played by phagocytes in defense against invading pathogens has been recognized since 1883. In that year, Metschnikoff, a Russian zoologist, reported that foreign particles injected into metazoans (in Metschnikoff's experiments, starfish larvae) were taken up by a population of "wandering mesodermal cells" that resided in interstitial tissues.¹ He postulated a crucial role in host defense for these wandering cells, which he named "phagocytes." Since Metschnikoff's discovery, hundreds of scientists studying dozens of species have reported thousands of studies on these cells, perhaps the most widely recognized of which are those of the eminent English physician Sir Colenso Ridgeon . . .