A low white blood cell count is a decrease in the cells in the blood that fight disease.
What's low in white blood cell count varies from one lab to another. This is because each lab sets its own reference range based on the people it serves. In general, for adults, a count lower than 3,500 white blood cells per microliter of blood is considered low. For children, an expected count depends on the age.
It's possible for some people to have white blood cell counts that are lower than what's usually expected and still be healthy. For instance, Black people tend to have lower counts than white people do.