IL6 and COVID-19: In another study, the number of T lymphocytes was shown to be inversely related to the levels of interleukin 6 (IL-6), interleukin 10 (IL-10), and tumor necrosis factor α (TNF-α), indicating that increased levels of pro-inflammatory cytokines may promote the depletion of T lymphocyte populations.[20] Another cause that may inhibit lymphocyte proliferation is the elevated lactic acid levels found in the blood of patients with severe COVID-19.[13,18] A study by Fischer et al examined the effect of lactic acid on the function of cytotoxic T lymphocytes.