CD38 expression is higher on CLL cells derived from splenic white pulp compared with splenic red pulp. (A) Sections of spleen stained for CD23 (shown in red) and CD38 (shown in green) at ×200 and ×630 original magnifications. In the colocalized images, yellow represents CD23⁺CD38^hi and red represents CD23⁺CD38^lo/− pixels. Although the ×200 views show that CD38 staining is apparent in both red and white pulp, the high-power views show that CD38 colocalizes with CD23⁺ tumor in the white pulp areas, whereas in the red pulp it does not and must be expressed by cells other than the tumor. (B) The table shows the percentage of CD23⁺CD38^hi and CD23⁺CD38^lo/− pixels in both white pulp (WP) and red pulp (RP) in sections of the 3 available spleens with high CD38 staining detectable in the white pulp. CD38 expression by the tumor cells is at least 10% greater in the WP compared with the RP (95% confidence interval shown in brackets). Significance was demonstrated using the χ² test.