Clinical Information

Several classes of ligands are capable of inducing blastogenesis and stimulating proliferation of lymphocytes in vitro, including plant mitogens (phytohemagglutinin [PHA], pokeweed [PWM], and concanavalin A [Con A]), bacterial products and superantigens (potent bacterial toxins that at low concentrations have the ability to activate large numbers of T cells), and phorbol esters. Cellular proliferation follows a complex series of signals that begins with engagement of lymphocyte surface receptors by a mitogenic or antigenic ligand. Subsequent signals, including gene activation and secretion of cytokines, result in synthesis of DNA and cell division. Measurement of mitogen-induced lymphocyte proliferation in vitro provides a semiquantitative assessment of total cell-mediated immunity.(1)

The proliferative responses to PHA and Con A involve T lymphocytes, and the response to PWM involves both T and B lymphocytes in a T-dependent manner. Diminished proliferative responses to lectin mitogens occur in a variety of primary and secondary immunodeficiency diseases including diseases that affect T lymphocytes, B lymphocytes, and T and B lymphocytes.(2)

Specific antigen recognition involves T-cell receptor recognition of specific peptide in the context of the appropriate MHC molecule on an antigen-presenting cell. T cells activate and proliferate in response to specific antigenic stimulus. The recall antigens (eg, Candida albicans and tetanus toxoid) are used to assess antigen-specific T-cell responses.

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 a.m. and noon, with no change between noon and afternoon. Natural killer (NK) cell counts, on the other hand, are constant throughout the day.(3) Circadian variations in circulating T-cell counts have been shown to be negatively correlated with plasma cortisol concentration.(4-6) In fact, cortisol and catecholamine concentrations control distribution and, therefore, numbers of naive versus effector CD4 and CD8 T cells.(4) It is generally accepted that lower CD4 T-cell counts are seen in the morning compared with the evening (7), and during summer compared to winter.(8) These data, therefore, indicate that timing and consistency in timing of blood collection is critical when serially monitoring patients for lymphocyte subsets.