Reactive lymphocytes in patients with COVID-19.

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. , 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.