CD4 and vitamin B12 deficiency: Intramuscular administration of vitamin B12 (in the form of methylcobalamin) to patients who have recently been diagnosed with vitamin B12 deficiency completely restores production of CD8+ T lymphocytes, returns the CD4+/CD8+ ratio back to normal levels, and restores CD3CD16+ and CD3CD57+ counts (both of which have high NK cell activity) (Figure 1), which results in restored NK cell activity [73].