What is the white stuff in zits?

We put this question to Dr Clifford Leen:

Clifford: Well don't think you should be squeezing the zits. The white stuff is made up of infection sometimes or sometimes just secretions from blocked-up sweat glands. Chris: What are the bugs doing? Are they infecting some kind of gland in the skin?Clifford: If it's infected there'll be germs in there but otherwise it'll just be secretions from a blocked sweat gland.Chris: The bottom line is, don't squeeze the zit but if you do it's just dead debris.