Human marginal zone b cells are not an activated b cell subset: strong expression of cd21 as a putative mediator for rapid b cell activation

In the human spleen the marginal zone (MZ) contains medium‐sized B cells with a distinct immunophenotype. A main function attributed to the MZ is its involvement in the primary response to blood‐borne antigens, in particular to thymus‐independent antigens type 2. In this study the presence of antigens related to activation and proliferation was evaluated in human spleens by immunohistochemical staining. It appeared that MZ B cells do not show interleukin 2 receptor expression, and are in G₀ phase of the cell cycle, as demonstrated by the lack of Ki‐67 reactivity. The most interesting finding is the high CD21 expression by MZ B cells in the absence of IgD expression. As the CD21 antigen has been shown to be involved in B cell activation in close linkage with IgM, it can be suggested that MZ B cells are particularly well equipped for rapid and easy activation in a primary immune response.