Polyclonal IgM has several fold increased binding to activated splenic B cells and dendritic cells (DC). (A) Splenic leucocytes, either un-activated (left panel) or activated for 48 hours (right panel) were interacted with mouse IgM at 4°C and evaluated for IgM binding using IgG anti-IgM (clone 11/41). Isotype monoclonal IgM with reactivity to KLH did not bind to activated leucocytes (data not shown). IgM binding to B cells was evaluated by blocking intrinsically expressed IgM with unlabeled IgG anti-IgM (clone 11/41). B cells and DC in spleenocytes were activated with LPS while T cells were activated with LPS and soluble anti-CD3. B cells, T cells, and DC were gated by CD19, CD3, and CD11c respectively. (B) Polyclonal IgM has increased binding to non-apoptotic and apoptotic activated splenic B cells and BMDC. Binding of IgM to annexin V positive cells (i.e. which are about 75% propridium (PI) positive) is not increased when compared to non-apoptotic cells. Splenic B cells were activated as in panel A while BMDC were activated for 48 hours with LPS (400ng). Activated cells were interacted with IgM and stained as in Fig 1A. B cells are gated as CD19+.