I'm going to be clear.   This sounds like it could be adenosylcobalamin-related, in that a deficiency of adenosylcobalamin triggers expression of the gene involved.   Adenosylcobalamin plays a direct role in regulating the immune system function and plays other roles as well.

Adenosylcobalamin stores last about three years (or so) in the body, and if there is any metabolic problem from birth around it, then a logical time for issues to show up is around age three, when a deficiency will start to have a domino effect on various processes in the body.   They way, and the degree, to which this occurs would largely depend on genetics / other genetic issues.   It could be mild and it could be life-threatening as well as everything in between.

The genetic panel of tests available for metabolic and neurological issues now available in Ontario includes a number of tests that will show genetics with significant implications for adenosylcobalamin stores:  cobalamin tests, MTHFR and ABCD.