There's no point in testing breast milk for blood components because it's well established that one of benefits of breastfeeding is the transference of immune properties (immunoglobulins).
A lot of people fail to logically compute how milk is produced. This is highly simplistic, but it conveys the process sufficiently for most new severely sleep deprived parents to understand.
Nutrients in by mouth --> digested in stomach --> absorbed by intestines --> enter the bloodstream --> blood circulates to mammary glands + stimulation by prolactin and oxytocin from hypothalamus + demands of hungry infant ==> milk production.
Leukocytes are present in all milk produced by mammals (humans and cattle included).
The World Health Organization would have a field day if the WTS ever dared to tell JWs that they couldn't breast feed their babies, or give them milk of any kind. I mean, even the Bible says that " like newborn babies, long for the pure milk of the word, so that by it you may grow in respect to salvation" (1Pet 2:2 ASV).
Besides, breast is best for the WTS, it means spending less money on formula and being able to put those savings in the Contribution Box™.