Neutrophils are a type of white blood cell (WBC), part of our immune system. The suffix “-penia” means “too few.” If the number of Neutrophils drops too low, we’re at risk of serious bacterial infections (see Differences Among Germs). On the average, we have around 4,000 neutrophils per unit of blood on a lab test. If the count drops under 1,000 clinicians get nervous, under 500 is dangerous, under 200 is most dangerous.
This is pretty rare. It may occur in some types of Leukemia (other types of leukemia cause extremely high WBC counts). More commonly, it happens from medicines. These include some types of cancer chemotherapy, medicines for hyperthyroidism (methimazole), for schizophrenia (clozapine), for epilepsy (valproic acid; Depakote®), and others. All sorts of common medicines can cause neutropenia in very rare cases. When we prescribe a medicine that has this as a significantly-possible side effect, we warn patients to seek care if they have a sore throat. If we see blisters of any sort in the throat, we order a blood test for WBCs. That tells us 100% for sure if this Neutropenia is present or possible.