“…We do not agree completely with this objection. In spite of the high hydrodynamic size of IgM, many studies showed a good correlation between the IgM level in the CSF and serum of normal subjects (4,5). We also had a good correlation between the CSF/plasma IgM ratio and the CSF/plasma albumin ratio in 57 tension headache subjects (r = 0.75, p < 0.01), suggesting that IgM physiologically penetrates the blood-brainbarrier in a way that can be compared with that of albumin (6).…”