Neutropenia is an abnormally low concentration of neutrophils (a type of white blood cell) in the blood. Neutrophils make up the majority of circulating white blood cells and serve as the primary defense against infections by destroying bacteria, bacterial fragments and immunoglobulin-bound viruses in the blood. People with neutropenia are more susceptible to bacterial infections and, without prompt medical attention, the condition may become life-threatening (neutropenic sepsis).

Neutropenia can be divided into congenital and acquired, with severe congenital neutropenia (SCN) and cyclic neutropenia (CyN) being autosomal dominant and mostly caused by heterozygous mutations in the ELANE gene (neutrophil elastase). Neutropenia can be acute (temporary) or chronic (long lasting). The term is sometimes used interchangeably with "leukopenia" ("deficit in the number of white blood cells").

Decreased production of neutrophils is associated with deficiencies of vitamin B12 and folic acid, aplastic anemia, tumors, drugs, metabolic disease, nutritional deficiencies (including minerals such as copper), and immune mechanisms. In general, the most common oral manifestations of neutropenia include ulcer, gingivitis, and periodontitis. Agranulocytosis can be presented as whitish or greyish necrotic ulcer in the oral cavity, without any sign of inflammation. Acquired agranulocytosis is much more common than the congenital form. The common causes of acquired agranulocytosis including drugs (non-steroidal anti-inflammatory drugs, antiepileptics, antithyroid, and antibiotics) and viral infection. Agranulocytosis has a mortality rate of 7–10%. To manage this, the application of granulocyte colony stimulating factor (G-CSF) or granulocyte transfusion and the use of broad-spectrum antibiotics to protect against bacterial infections are recommended.

Signs and symptoms of neutropenia include fever, painful swallowing, gingival pain, skin abscesses, and otitis. These symptoms may exist because individuals with neutropenia often have infection. In neutropenia, latent infections that are normally controlled by an intact immune system may undergo reactivation; a notable example is dental infections of the jaws originating from necrotic dental pulp.

Children may show signs of irritability and poor feeding. Hypotension has also been observed in individuals with this condition.

The causes of neutropenia can be divided between problems that are transient and those that are chronic. Causes can be divided into these groups:

Severe bacterial infections, especially in people with underlying hematological diseases or alcoholism, can deplete neutrophil reserves and lead to neutropenia. Gram-positive bacteria are present in 60–70% of bacterial infections. There are serious concerns regarding antibiotic-resistant organisms. These would include as methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant Enterococcus (VRE).

Nutritional deficiencies, such as deficiency in vitamin B12, folate, copper or protein-calorie malnutrition are associated with chronic neutropenia. However, nutritional deficiencies are usually associated with decreases in other cell lines (multiple cytopenia or pancytopenia) rather than isolated neutropenia.