 In 1887, Élie Metchnikoff observed that leukocytes isolated from the blood of various animals were attracted towards certain bacteria.This attraction was soon proposed to be due to soluble elements released by the bacteria  (see Harris for a review of this area up to 1953).Peter Ward, Elmer Becker, Henry Showell, and colleagues showed that these elements were made by a variety of growing gram positive bacteria and gram negative bacteria and were of low molecular weight, i.e.