IL4 and bacterial infectious disease with sepsis: The interaction of protein L with human mast cell progenitor FcεRI+ cells leads to the synthesis and release of IL-4 and IL-13 (Genovese et al., 2003), and here we find that release of IL-4 from neutrophils is also triggered by protein L. Recently, protein L was hypothesized to be involved in a fatal case of monomicrobial F. magna bacteremia, causing toxic shock-like symptoms in a patient (Rosenthal et al., 2012).