I guess they're referring to the brain (and, by extension, the eyes!) being an "immunoprivileged zone" [1]... the brain-blood-barrier makes a lot of "everyday" stuff for the rest of the body pretty complex affairs for everything CNS related.

[1] https://en.wikipedia.org/wiki/Immune_privilege

Anything immunotherapy in cancer treatment is extremely targeted. Early trials with new drugs include people getting cooked to death as the immune system and in particular the wrong interleukins get over-stimulated. 'Bolstering of the immune system' sounds good, but in general the immune system is a fine balance between doing the job against pathogen/cancer but not well that it is erring on being nonspecific and attacking self, i.e. autoimmunity. Unless otherwise noted, non-specific 'bolstering' in a tissue like the brain is likely not a good thing.