The loss of axons is certainly the cause of long term disability in MS patients, however short term loss of function can be caused by the improper conduction of denuded axons. If left to a toxic environment the axons themselves will eventually die.
The theory behind the functional recovery that follows treatment with drugs such as Campath is that the axons are remyelinated and therefore do not continue to die.
In addition the Brain is capable of rewiring itself past injured areas, a feature known as plasticity. It may be that this plays a part in functional recovery.
Finally some axon regeneration may be occurring (although not proven). The following is an extract from Alasdair Coles talking about Campath.
We imagined that Campath-1H treatment of multiple sclerosis would prevent further deterioration but would do nothing for damage already acquired. Quite unexpectedly however, most patients treated using a single dose of Campath-1H show a steady improvement in their disability over the next 12-24 months. This was encouraging but difficult to understand. The brain was being encouraged to repair itself; but why?
This project tests one possible explanation: that cells of the immune system are altered after Campath-1H treatment and they travel to the brain to release factors that encourage repair of nerve fibres.
Do not go gentle into that good night. Rage, rage against the dying of the light.