From the Hematology of Infancy and Childhood (5th Edition):
"Individual patients may be characterized as having mild neutropenia with neutrophil counts of 1000 to 1500 cells/uL, moderate neutropenia with counts of 500 to 1000 cells/uL, and severe neutropenia with counts generally fewer than 500 cells/uL. This stratification is useful for predicting the risk of pyogenic infections, because only patients with severe neutropenia have increased susceptibility to life-threatening infection, particularly if the neutropenia persists for more than a few
days. Endogenous bacteria are the most frequent invaders, but colonization with a variety of organisms of nosocomial origin is also common.
Susceptibility to bacterial infection, even with severe neutropenia, is quite variable. For example, some patients with chronic neutropenia due to autoantibodies do not experience serious infections over a period of many years even with neutrophil counts as low as 200 cells/uL (or even lower), most likely because these individuals have normocellular bone marrow."