MCs are spatially associated with areas of dense CD40 stromal expression. In BM biopsies with high CD40 stromal expression, tryptase⁺ MCs were observed within neoplastic lymphoid aggregates and showed a higher density in areas with stronger CD40 expression. Such association is detectable by immunohistochemical analysis for MC tryptase and CD40 (brown signals) on BMB serial sections (A) and by double-marker immunofluorescence on confocal microscopy for the same markers (CD40, green signal; tryptase, red signal) (C). Immunohistochemistry, STREPTavidin–biotin–peroxidase complex method, DAB chromogen; original magnification ×400. Immunofluorescence with Alexa-488 and Alexa-568 fluorochromes; original magnification ×200. Microphotographs are relative to 2 representative cases with high (score 3) and low (score 1) CD40 stromal expression. (B) Significant positive correlation between the degree of infiltrating tryptase⁺ and CD40⁺ BMSCs in SMZL patients.