Will taking multivitamins boost my immune system?

No...: ...Not unless you have significant vitamin deficiency to start with.

Not usually: There is some evidence that vitamin d3 deficiency may impact immune function. Folic acid deficiency may be another potential factor but the latter is rather uncommon except in alcoholics in this country. If you have low serum vitamin d3 level, by all means, take some supplement. The amount of vit d3 in a multivitamin preparation however is too enough for most people.