The case of a male with a late-onset type of combined immunodeficiency and a chronic progressive encephalopathy is reported. In this case serum IgM was undetected but cerebrospinal-fluid (C.S.F.) IgM was demonstrated which appeared to be similar to the normal polymeric form of IgM. This occurrence of IgM in the C.S.F. did not appear to be due to disruption of the blood/brain barrier, since other large-molecular-weight serum-proteins such as alpha-2-macroglobulins and fibrinogen were absent from the C.S.F. Therefore, it appears that the C.S.F. IgM may have been produced by lymphoid cells in the brain.
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