Élie Metchnikoff
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. below 3600 Dalton (unit)s. Further studies by Schiffmann and colleges found that cultures of growing Escherichia coli released oligopeptides of between 150 and 1500 daltons that appeared to have a free Carboxylic acid group but not a free Amine group.