Janet, I don't really understand it myself, but in broad terms it's an easily identifiable marker in the blood that can be tracked to measure immune response. Not everyone is positive for HLA2, and for this particular trial it's a prerequisite as it's the way the researchers will measure response to the vaccine. HLA typing is used in organ transplants also...Here's a link to a more detailed explanation:
http://www.merck.com/mrkshared/mmanual/sec...ter149/149b.jsp
Love, Lolly