I've just spent a pleasant afternoon in the library reading around
the subject of lipid neurochemistry and thought I'd share a couple
of interesting things that I've learned about myelin and myelination.

from 'Essentials of Neurochemistry' edited by G.C. Wild and E.C. Benzel

page 8 [... sequence of myelination.....]
"The sequence of myelination corresponds roughly to phylogenesis, or
the evolutionary sequence. The peripheral nervous system (PNS) myelinates
first, followed by the spinal cord. The cerebral cortex ... much later.
Arcuate
fibers, which connect different portions of the cerebral cortex to each
other, are the last to be myelinated .... [and are probably] not completely
invested with myelin until the third decade of life".

page 11 [... immunity, blood-brain barrier...]
"The most characteristic lipid of myelin is another sphingolipid, cerebroside,
.... large amounts are present in myelin and much of this is synthesized
after the blood-brain barrier has formed. This fact, which is also true of
many other brain molecules, implies that the body's immune system does
not fully recognize cerebrosides (and these other molecules) as 'self', and
therefore is capable of mounting an immune response to them."

Ordinarily, cerebroside will not appear in the blood as its turnover occurs
entirely in the white matter of the central nervous system.

"Sphingomyelin is not peculiar to brain, and may therefore be recognized
as 'self' by the immune system. Many negatively charged molecules,
and especially negatively charged lipids, do not serve well as antigens
[many 'self' molecules are negatively charged so perhaps immune responses
are generally repressed against them]. .... the major lipid autoantigen of
myelin is cerebroside.... if myelin is injected into another animal of the
same species it will initiate the production of antibodies [even if injected
into an identical twin of the animal from which the molecule was taken].
Some immune responses are specific for myelin proteins, but some are
specific for cerebroside.

Well I hope not everyone on the list knew all of that already !
Best wishes
Jo