BTK and myeloid sarcoma: While we focus here on effects of BTKi on peripheral B-cell responses and B-cell:T-cell interactions that are thought to be involved in the development of new MS relapses, BTK is also expressed on myeloid cells and the known capacity of BTKi to limit myeloid cell activation (with the potential to limit consequent T-cell activation) may also be relevant to the therapeutic mode of action of BTKi on MS relapses.