In correcting this paper an extra line of text was inadvertently inserted at the top of the second column on page 219. The continuation of text from column 1 to column 2 on this page should read:. blood-brain barrier (BBB), which is impermeable to many soluble substances (i.e. immunoglobulins), and restricts the. Line 2 of column 1 on page 220 should read:. like molecule (3), can be induced to express class II major. © 1989 Oxford University Press.