FCGR2A and Absence of circulating granulocytes: Our findings of lower serum IgM levels in patients who developed LON and lower serum IgM levels in association with the FCGR3A V allele, together with a longer time to flare, indicate that the phenomenon of LON is of a different nature than drug-induced agranulocytosis and that LON is likely explained by biological mechanisms of rituximab action modified through FCGR genotypes (i.e., a more pronounced B-cell depletion).