Monocytes migrate into the tissues where they become macrophages, with specific characteristics depending on their tissue localization.
Monocytopenia can increase the risk of infection, and it can indicate poor prognosis in patients with acetaminophen-induced hepatic damage and thermal injuries. Peripheral blood monocytopenia does not usually indicate a decrease in tissue macrophages; in some cases it can be associated with impaired granuloma formation in response to infections.
Monocytopenia can result from
Transient monocytopenia can occur with endotoxemia, hemodialysis, or cyclic neutropenia.
A severe deficiency or absence of monocytes can occur in patients with mutations of the hematopoietic transcription factor gene, GATA2. Dendritic cells are decreased, and there may also be lymphocytopenia (mainly natural killer and B cells).
Despite near-absence of circulating monocytes, tissue macrophages are usually preserved. Also, immunoglobulin levels are usually normal even when circulating B cells are depressed. Bone marrow is hypocellular and can show fibrosis and multilineage dysplasia. Karyotypic abnormalities, including monosomy 7 and trisomy 8, may be present.
Infections with Mycobacterium avium complex (MAC) or other nontuberculous mycobacterial infections are common (MonoMAC syndrome). Fungal infections (ie, histoplasmosis, aspergillosis) also are typical. Infections with human papillomavirus (HPV) may occur with subsequent risk of progression to secondary cancers. There is a high risk of progression to hematologic disorders (myelodysplasia, acute myeloid leukemia, chronic myelomonocytic leukemia, lymphomas) with a resulting poor prognosis.