A few years ago ProHealth ran an article about transfer factor from a researcher at Duke Med Center named Aaron White. He now has a book out about transfer factor that is in its second edition. I haven't read the second edition but the first edition contained everything anyone could possibly want to know about transfer factor, including how they work, how much to take, what to expect and so on.
Also, there are other options for immune weaknesses available now that are good in different ways than transfer factors. Doing a search for "thymus protomorphogens" will yield interesting results. Protomorphogens are molecules that trigger specific organs (in this case the thymus, in which T-cells mature) to rebuild and work harder.
As other posters have pointed out, transfer factors don't seem to "fix" the immune problems that leave some of us vulnerable to infections. Rather, they help the body go after and kill whatever pathogens are in there. They do boost immune system health, but not enough to keep all people from relapsing if they stop taking them. Protomorphogens for thymus, thyroid, adrenals, and even the pituitary and hypothalamus seem to be promising adjuncts for treating basically any condition with an immune component.
Hope this helps!!
PS - The book is called "A Guide to Transfer Factors and Immune System Health, 2nd Edition" Again, I haven't read it but the first edition was excellent.