PRF1 and infection: Interestingly, by day 4 post-TBI, a time-point where the frequency of perforin-positive CD3−CD56+ NK cells correlates positively with GCS score, the percentage of perforin-positive CD3−CD56+ and CD56DIM NK cells is significantly reduced relative to healthy controls (32), a decline that may contribute to the increased susceptibility of the hospitalized TBI patient to infection.