Myelin basic protein and its free and bound antibodies in cerebrospinal fluid. All three must be determined on each specimen.

Myelin basic protein (BP) in the cerebrospinal fluid (CSF) is an important marker of brain damage, especially of white matter, but low or "normal" values can be misinterpreted if the CSF is not also examined for free and bound antibodies to BP. BP has many epitopes (antigenic determinants) and is very susceptible to fragmentation by proteolytic enzymes that are frequently very active in CSF, especially in patients with neurological diseases. This combination of factors permits BP to be degraded as it is released from myelin and to allow antibodies to those epitopes destroyed by enzymatic action to persist free in the CSF. Other free anti-BP antibodies may simply represent antibody excess. The relatively frequent formation of soluble antigen-antibody complexes with other epitopes on BP permits the existence of anti-BP antibodies bound to BP also in CSF. In addition to these factors that contribute to low values of free BP in CSF, if the analyses are not performed promptly on CSF collected in plastic tubes (polystyrene but preferably polypropylene), the "natural" adhesiveness of BP (immediately to glass, slowly to plastic) can remove it from the CSF, resulting in artifactually low values which can also be misinterpreted as "normal".