CD27 and systemic lupus erythematosus: Also, antigen-experienced B cells [including atypical memory (AtMs), double negative 2 (DN2), or age-associated B cells (ABCs)] which share canonical phenotype and are identified as CD24-CD20hi, IgD-CD27-, or CD19+CD21low B cells are frequently detected in SLE patients [20, 22, 23].