Reactive lymphocytes in patients with COVID‐19

Abstract

From 23 January to 27 February 2020, Singapore had 96 COVID-19 cases confirmed by real time reverse transcriptase-polymerase chain reaction (RT-PCR) for SARS-CoV-2. We examined the peripheral blood films of 32 patients and found reactive lymphocytes as shown in the top images in 23 cases (72%). This is in stark contrast to the pattern with coronavirus responsible for the 2003 SARS outbreak where reactive lymphocytes of this type were not present in a review of 185 SARS cases in Singapore and were present in only 15·2% of 138 cases in Hong Kong.1, 2 Reactive lymphocytes are commonly seen in other viral diseases such as dengue fever and infectious mononucleosis. They have varied morphological features. The most common subtype seen in our COVID-19 patients displayed a distinctive abundant pale blue cytoplasm that often abuts adjacent red blood cells (top left and right). Strikingly, lymphoplasmacytoid lymphocytes were present in 16 out of 23 patients (bottom images: left, right and centre). These are small mature lymphocytes with condensed chromatin and an eccentric nucleus, occasionally with a paranuclear hof. Lymphoplasmacytoid lymphocytes are also seen in dengue fever and in several B-cell non-Hodgkin lymphomas. Reactive lymphocytes of both types can coexist in a single peripheral blood film in COVID-19 patients.