Various B cell subsets were identified using FACS, CyTOF, scRNA-seq, and CITE-seq in COVID-19 patients, including naïve B cells (TCL1A+SELL+), atypical memory B cells (atMBC, ITGAX+FCRL5+), activated B cells (ABC, CD21low CD27+ CD10-), memory B cells (MBC, CD21+ CD27+ CD10-), etc. According to their class switching states, memory B cells also comprised class-switched (IgD- CD27+) and non-class-switched (IgD+ CD27+) subsets, where the class-switched memory B cells are thought to have undergone affinity maturation through GC reactions. This evidence concerns the gene CD27 and COVID-19.