The milk analogy's a good one.
Whether leukocytes remain in commercially produced milk appears to depend on the way it is treated.
"Before pasteurization, most homogenized milk undergoes a centrifugal filtering process called clarification, which removes leukocytes. In Massachusetts, the whole milk was passed through a milk filter rather than clarified, so that leukocytes were not removed." (from a medical article about listeriosis infection) - http://content.nejm.org/cgi/content/full/312/7/404#R10