St. John's wort as you know, is a poisoner's dream potion. Administered after dark its photochemistry changes deadly in daylight. Ok, maybe you don't have that factoid on the tip of your tongue. Unless things have suddenly changed, you aren't at risk for needing a transplant so increased immunity would be helpful in all probability.
If you don't mind, why are you considering taking St. John's wort? Where did you learn about this particular use for it? Oh, yes it is a photoreactive poison, just keep to recommended dosing if you take it, avoid unknown dosages like teas and fresh or dried plants or their parts.
Many uses have been found for SJW from immunity improvement to antidepressants and Parkinson's treatment. I find many conventional drugs used to treat these issues, wanting. To that end, SJW may be worth a try. IBS or irritable bowel syndrome is possibly treated with it or make it worse and a number of other medications interact with it. Involve or at least inform your doctor of your plans. Look up St. Johns wort in Wikipedia to see the major drug classes you may be in conflict with. Interactions could be serious, especially neuroleptics and other psychoactive drugs like anti anxiety drugs, mood stablizers and such. Even antibiotics and possibly cheese can be intertwined in increasing/lowering the strength. Yah cheese is to be taken carefully with certain drugs including SJW.
Please let me know what you are looking to use SJW for, sounds interesting.
LL