An abscess is a sore beneath the skin which is filled with pus and surrounded by a wall of mucus membrance called a capsule. In most species, these things are primarily a nuissance and are generally very treatable. However, with rabbits, things aren’t so simple (not that that should be surprising).
You see, in most other species, the pus inside an abscess is broken down by enzymes in the body, which keeps the fluid fairly liquid. Thus, if the abscess breaks outward, it can drain fairly easily and then heal from the inside out. With rabbits, the picture is quite different. They, unfortunately, don’t possess this enzyme, which means the fluid in the abscess remains thick and pasty, nearly the consistency of tooth paste. Thus, treatment of rabbit abscesses is far more complicated, since the poorer drainage leads to greatly increased chance of reoccurrence and slower healing process, which makes it necessary to flush the wound with an antiseptic.
Moreover, if the abscess is allowed to persist, it may break inward, causing a sudden bacterial flux that can kill the rabbit. But wait, it gets better! For most animals, it’s fairly trivial to give oral antibiotics which can reduce the chance of severe infection. However, due to the digestive system of rabbits (which contains a variety of necessary bacteria which play a key role in digestion), certain classes of antibiotics cannot be used, such as penicillin derivatives, lest the rabbit develop GI stasis (basically, the digestive system can shut down). Consequently, you must either use sulpha-based drugs or perform subcutaneous injections of other drugs (although there’s still a chance of the rabbit developing GI stasis, injections make this significantly less likely).
Finally, frequent abscesses in rabbits can often indicate a more serious underlying condition, such as a low-grade Pasteurella infection, which further complicates treatment, and may require long term antibiotic therapy (which, again, carries the risk of GI stasis).
Incidentally, our own rabbit, Chloe, has been on and off long-term subcutaneous penicillin, and it has worked remarkably well in treating her abscesses, while not causing any additional health concerns. If you have a rabbit which has abscess problems, you should talk to your vet about subcu penicillin as a treatment option.