Lenalidomide and immunity - does it get better?

Hi, the short answer is "yes".

The stem cell transplant will have left your sister with an immune history like a newborn baby's. For example, past vaccinations will count for nothing. That she has had a couple of infections after the transplant will help to rebuild her immune history. She will also be revaccinated at some point.

As she recovers your sister's immune system will gradually improve, although the "immune system" actually consists of many things: neutrophils, basophils etc, and many different kinds of T-cells.

You mention that your sister is on lenalidomide..The next bit is from ChatGPT.

"Despite some immune-boosting features, the net result of long-term lenalidomide maintenance is:Increased susceptibility to opportunistic infections (e.g., viral reactivations, fungal infections, PJP). Prophylactic antivirals (like acyclovir) and sometimes antibiotics are used to mitigate this."

I am on maintenance myself. Yes, I have had a couple of infections since going into remission a couple of years ago, but not frequently. I just keep away from people when my neutrophils have been low, wash my hands frequently, check that restaurants etc have good hygiene standards. Things like that.