Germinal centers and BCL6-expressing Tfh and B cells are decreased in the lymph node and spleen of a patient with P-HLH. Panel showing multiplex immunofluorescence (CD3, red; CD19, green; BCL6, yellow; and DAPI, blue) images from (A) lymph node and (B) spleen of controls (a 1-month-old infant and a 3-year-old) and a patient with active P-HLH (aged 2 months). Primary follicles present in lymph nodes of both controls. Germinal centers not prominent in lymph node of younger control but present in older control. Lymph nodes of the patient with HLH lack primary follicles and germinal centers. The spleen of the control demonstrates areas of distinct white pulp, whereas the spleen of the patient with HLH lacks white pulp.