Clinical Information

Natural killer (NK) cell cytotoxicity includes spontaneous cytotoxicity against target cells, typically lacking surface major histocompatibility complex class I expression, cytokine-stimulated cytotoxicity (also called lymphokine-associated cytotoxicity), and antibody-dependent cellular cytotoxicity. NK cell cytotoxic function is a key aspect of innate immunity and critical to maintenance of immune function, particularly with regard to viral infections (herpes viruses) and tumor surveillance. Cytotoxic NK cells typically express CD16 at high levels and CD56 at very low levels (CD16+56+) and account for the majority of circulating NK cells, while cytokine-producing NK cells express CD56 abundantly but little to no CD16 (CD56brightCD16±) expression. Cytotoxic NK cells contain cytotoxic granules expressing proteins such as perforin, granzyme A and B and granulysin, which participate in the effector cytotoxic function.

T- and B-Cell Quantitation by Flow Cytometry:

Normal immunity requires a balance between the activities of various lymphocyte subpopulations with different effector and regulatory functions.

Different immune cells can be characterized by unique surface membrane antigens described by a cluster of differentiation nomenclature (eg, CD3 is an antigen found on the surface of T lymphocytes). Abnormalities in the number and percent of T (CD3), T-helper (CD4), T-suppressor (CD8), B (CD19), and natural killer (CD16+CD56) lymphocytes have been described in a number of different diseases. In patients who are infected with HIV, the CD4 count is measured for AIDS diagnosis and for initiation of antiviral therapy. The progressive loss of CD4 T lymphocytes in patients infected with HIV is associated with increased infections and complications.

The Public Health Service has recommended that all HIV-positive patients be tested every 3 to 6 months for the level of CD4 T lymphocytes.

The absolute counts of lymphocyte subsets are known to be influenced by a variety of biological factors, including hormones, the environment, and temperature. The studies on diurnal (circadian) variation in lymphocyte counts have demonstrated progressive increase in CD4 T-cell count throughout the day, while CD8 T cells and CD19+ B cells increase between 8:30 am and noon, with no change between noon and afternoon. NK cell counts, on the other hand, are constant throughout the day.(1) Circadian variations in circulating T-cell counts have been shown to be negatively correlated with plasma cortisol concentration.(2-4) In fact, cortisol and catecholamine concentrations control distribution and, therefore, numbers of naive versus effector CD4 and CD8 T cells.(2) It is generally accepted that lower CD4 T-cell counts are seen in the morning compared with the evening,(5) and during summer compared to winter.(6)

These data, therefore, indicate that timing and consistency in timing of blood collection is critical when serially monitoring patients for lymphocyte subsets.