Scientists in early 2007 put forth the theory that the drug acted in a different fashion in humans as compared with the laboratory animals in which the drug was first tried. The severe reactions in humans could have only occurred, they believe, in those with memory T lymphocytes. Animals raised in a sterile lab would presumably have no 'memory' of previous illnesses, thus would not exhibit the severe reactions that occurred in the human subjects. However this is a misunderstanding of the research: the research says that lab animals studied have fewer memory T cells than humans, and that stimulation through the CD28 receptor alone in memory T cells causes them to infiltrate organs and also activates them.