ABSTRACT
OBJECTIVE: Complete blood count parameters are frequently altered in COVID-19 patients. Leucopenia and lymphopenia are the most common findings. This is not specific to COVID-19 as similar alterations are found in various other viral infections. This work is intended to summarize the evidence regarding white blood cell and lymphocyte subset alterations in COVID-19 and their clinical implications. MATERIALS AND METHODS: A PubMed search was conducted to identify relevant original studies. Articles not available in English or referring exclusively to pediatric patients were excluded. The study was designed as a narrative review from its inception. RESULTS: Complete white blood cell number and lymphocytes may be reduced in COVID-19 patients. Circulating CD4+ cells (helper T lymphocytes), CD8+ cells (cytotoxic T lymphocytes), regulatory T cells and natural killer (NK) cells may be reduced, with a greater reduction observed in critically ill patients. CD4+ and regulatory cell deficiencies may contribute to the cytokine storm and subsequent tissue damage observed in severe COVID-19 infection. NK and CD8+ cell deficiency might delay infection clearance. These aberrations of cellular immunity may contribute significantly to the pathogenesis of the disease. Alterations observed in monocyte function can also be implicated as they are effector cells responsible for tissue damage and remodeling. B cell dysfunction and maturation abnormalities have also been reported, suggesting that the virus also impairs humoral immunity. CONCLUSIONS: Lymphocyte subset abnormalities may be useful prognostic biomarkers for COVID-19, with circulating CD8+ cell count being the most promising as a predictor of severe disease requiring mechanical ventilation and mortality.