Consequently, pronounced lymphopenia (i.e., low numbers of CD3+ and CD4+ T cells, particularly of the Th1-type and regulatory T cells (Tregs); very low numbers of CD8+ T, B and natural killer (NK) cells) and T and NK cell exhaustion (e.g., reduced expression of CD26; increased expression of T-cell immunoglobulin and mucin domain-3 (TIM-3) and programmed cell death protein (PD)-1) have been recorded [5,6,7,8]. Here, HAVCR2 is linked to lymphopenia.