I was on methotrexate for a long time as an arthritis treatment. It is a chemotherapy agent,
i had to take calcium foliage while I was on this drug.
My understanding is that the red blood cells get enlarged or out of shape reducing surface area for oxygen and iron uptake etc. The folinates ( iron) restore the red cells to a more normal size/shape. It isn’t to do with white cells or immune system.
Ultimately it prevents anaemia.