Mononuclear Phagocytes (Monocytes)

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago

Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at LSU (Shreveport) Department of Allergy and Immunology

Mononuclear phagocytes circulate in the blood as monocytes and reside in all tissues of the body as macrophages.

Macrophages differentiate into specialized forms in different tissues remembered by the mnemonic KOMA:

Kupffer cells (liver)
Osteoclasts (bone)
Microglia (CNS)
Alveolar macrophages (lung)

Monocytes turn into macrophages once they enter tissues. Dendritic cells are not the same as macrophages. Monoblasts give rise to both macrophages and dendritic cells.

Monocytes express CD14 and CD16. They migrate to peripheral tissues via VLA-4/VCAM-1 interactions. Monocytes express CD14, CD11b/CD18 (Mac-1), and CD36.

Monocytes are CD45+ CD14+ cells. Granulocytes are CD45+ CD15+ cells.

CD14 is present on macrophages. CD14 is the first described pattern recognition receptor (PAMP receptor) and it binds to bacterial LPS.

TLR4 (for LPS)

CD14

CD16 is FcγRIII, a low-affinity Fc receptor for IgG. CD16 is found on NK cells, macrophages, and neutrophils.

Fc gamma receptors for IgG are CD 16, 32, 64 (double). Fc gamma receptors for IgG (CD 16, 32, 64) are labeled in "reverse" order:

CD 16 - FcR III - low-affinity

CD 32 - FcR II - intermediate-affinity

CD 64 - FcR I - high-affinity

CD16 and CD56 are present on NK cells.

Fc II receptors:

CD 32 - Fc gamma II

CD 23 - Fc epsilon II, Fc receptors for IgE

Hemophagocytic lymphohistiocytosis (HLH)

Hemophagocytic lymphohistiocytosis (HLH) or macrophage activation syndrome is an uncontrolled activation of macrophages with increase in circulating cytokines. This potentially life-threatening disease is

characterized by fever, hepatosplenomegaly and high ferritin. Hemophagocytosis can often be seen on bone marrow

biopsy.

In boys with X-linked lymphoproliferative (XLP) syndrome (Duncan’s syndrome), HLH is often a fatal complication of Epstein-Barr virus (EBV) infection.

Related reading

Interferon regulatory factor 8 (IRF8) Mutations and Human Dendritic-Cell Immunodeficiency. NEJM, 2011.

Published: 05/18/2009

Updated: 09/20/2011