This study investigates the dysregulation of immune response in patients with COVID-19 at Tongji Hospital in Wuhan, China, from January 10 to February 12, 2020. The researchers analyzed demographic and clinical data, laboratory examinations, and lymphocyte subsets of 452 confirmed cases. They found that severe cases (n=286) had lower lymphocyte counts, higher leukocyte counts, and a higher neutrophil-to-lymphocyte ratio (NLR), as well as lower percentages of monocytes, eosinophils, and basophils. Severe cases also showed elevated levels of infection-related biomarkers and inflammatory cytokines. T cell counts were significantly decreased, with a more pronounced impairment in severe cases. Helper T (Th) cells and suppressor T cells were below normal levels, and the percentage of naive Th cells increased while memory Th cells decreased. Regulatory T cells were also lower, particularly in severe cases. The study suggests that the novel coronavirus primarily affects lymphocytes, especially T lymphocytes, and that monitoring NLR and lymphocyte subsets can aid in early screening, diagnosis, and treatment of COVID-19.This study investigates the dysregulation of immune response in patients with COVID-19 at Tongji Hospital in Wuhan, China, from January 10 to February 12, 2020. The researchers analyzed demographic and clinical data, laboratory examinations, and lymphocyte subsets of 452 confirmed cases. They found that severe cases (n=286) had lower lymphocyte counts, higher leukocyte counts, and a higher neutrophil-to-lymphocyte ratio (NLR), as well as lower percentages of monocytes, eosinophils, and basophils. Severe cases also showed elevated levels of infection-related biomarkers and inflammatory cytokines. T cell counts were significantly decreased, with a more pronounced impairment in severe cases. Helper T (Th) cells and suppressor T cells were below normal levels, and the percentage of naive Th cells increased while memory Th cells decreased. Regulatory T cells were also lower, particularly in severe cases. The study suggests that the novel coronavirus primarily affects lymphocytes, especially T lymphocytes, and that monitoring NLR and lymphocyte subsets can aid in early screening, diagnosis, and treatment of COVID-19.