In situ phenotyping of FL grade 1/2 on formalin-fixed, paraffin-embedded sections using multiplex immunohistochemistry. (A) The normal architecture of the LN is effaced by neoplasia follicles of malignant B cells (CD20, clone L26, cyan) sustained by CD21⁺ FDCs (clone EP3093, magenta). Tumor cells strongly express BCL2 (clone SP66, green). Tumor-infiltrating T cells include a high number of CD4⁺ T cells (red) in interfollicular and intrafollicular areas, whereas CD8⁺ T cells (clone C8/144B, orange) are less numerous and usually observed at the follicle border. (B) Detection of Tfh and Tfr subpopulations in FL. Tfh cells are mostly observed in neoplastic follicles and coexpress CD4 (red), PD1 (clone NAT105, yellow), and CXCR5 (clone D6L3C, magenta). High magnification of follicle area shows Tfr positive for CD4 (red), CXCR5 (magenta), and FOXP3 (clone EP340, green).