In the spirit of noding what you don't know… Here is a very simplistic explanation, as culled from the unfathomable morass that comprises the most basic explanations now available to man.
When your body starts to make antibodies*, it makes a lot. But eventually, you're going to get rid of all the antigens**, and then you're likely to have a surplus of antibodies - now useless. But your body is prepared - it has already made a second set of meta-antibodies (not a technical term) to get rid of the extra antibodies by mimicking the antigens. As you might have guessed, these are called 'anti-idiotypic antibodies'. To keep things running along smoothly, the production of anti-idiotypic antibodies is automatic, but blocked by the presence of the antigens.
You might wonder what the term 'anti-idiotypic antibodies' actually means (I know I do). Well, it turns out that an idiotypic antibody is "immunologically defined by reactivity with more than one anti-idiotype that recognizes an idiotypic determinant or idiotope within a given idiotype". Yes, you read that right the third time - it turns out that the definition of idiotype is recursive.
You can also get cool things like 'anti-anti-idiotype antibodies'. (Just read anti- as meta- and you'll get the idea).
All this was started by good old Niels K. Jerne in 1974, with his book Toward a network theory of the immune system. These days you're most likely to hear about anti-idiotypic antibodies in connection with cancer research, but they pop up all over the place.
As you have no-doubt noticed, I don't actually know what I'm talking about. I have no idea how this stuff fights cancer, immunizes against cocaine, treats rheumatoid arthritis, or does any of the other cool stuff it does. I have no idea how the whole 'T-cell issue' works into this. I don't know what exactly the anti-idiotypic antibodies do with/to the antibodies to get rid of the extra antibodies. I don't know how the body makes the anti-idiotypic antibodies. In fact, if someone out there does understand all this stuff, here's an excellent chance to earn a C!, up your node count, and best of all, earn the undying respect of your peers. (But please, keep it simple enough that we can understand it).
* The good guys, used for fighting germs and other Bad Things. Unfortunately, each antibody is only good for one type of Bad Thing.
** the aforementioned Bad Things.