signal to other cells to differentiate into the proper leukocytes

Shroom,

First thing that comes to mind is "Cytokine". Second thing is "it's not that easy".

By doing a search using >differentiate leukocytes cytokine<
as a search term will get some interesting results.

Here's one for the less motivated
http://microvet.arizona.edu/Courses/MIC419/Tutorials/immunityrules.html


Labrat