Expert Opinion :
Natural killer {NK} cells are lymphocytes of the innate immune system. They predominantly act against viral infections and in tumor cell surveillance. The viruses that seem to be best targeted by NK cell mediated defenses are those of the Herpesvirus family. There are 2 major types of NK cell deficiencies {NKD}. Classical NKD {CNKD} is defined as an absence of NK cells and their function among peripheral blood lymphocytes. Functional NKD {FNKD} is defined as the presence of NK cells within peripheral blood lymphocytes, having defective NK cell activity. To diagnose CKND, NK cells should be =1 percent of peripheral blood lymphocytes. Most patients present with varicella, cytomegalovirus {CMV}, Epstein Barr virus {EBV} and herpes simplex virus {HSV} infections. This child has not had such infections. Thus it is unlikely to be NKD. Her repeat lymphocyte subset after 3 months was normal.