Monocytes are a type of white blood cells.
They are the largest of all leukocytes.
They are part of the innate immune system of vertebrates including all mammals, birds,
reptiles, and fish.
They are amoeboid in shape, having clear cytoplasm.
Monocytes have bean-shaped nuclei that are unilobar, which makes them one of the types
of mononuclear leukocytes.
Monocytes constitute 2% to 10% of all leukocytes in the human body.
They play multiple roles in immune function.
Such roles include: replenishing resident macrophages under normal states, and in response
to inflammation signals, monocytes can move quickly to sites of infection in the tissues
and divide/differentiate into macrophages and dendritic cells to elicit an immune response.
Half of them are stored in the spleen.
Monocytes are usually identified in stained smears by their large kidney shaped or notched
nucleus.
These change into macrophages after entering into the tissue spaces.
Physiology Monocytes are produced by the bone marrow
from precursors called monoblasts, bipotent cells that differentiated from hematopoietic
stem cells.
Monocytes circulate in the bloodstream for about one to three days and then typically
move into tissues throughout the body.
They constitute between three to eight percent of the leukocytes in the blood.
Half of them are stored as a reserve in the spleen in clusters in the red pulp's Cords
of Billroth.
In the tissues, monocytes mature into different types of macrophages at different anatomical
locations.
Monocytes are the largest corpuscles in the blood.
Monocytes which migrate from the bloodstream to other tissues will then differentiate into
tissue resident macrophages or dendritic cells.
Macrophages are responsible for protecting tissues from foreign substances, but are also
suspected to be important in the formation of important organs like the heart and brain.
They are cells that possess a large smooth nucleus, a large area of cytoplasm, and many
internal vesicles for processing foreign material.
Monocytes and their macrophage and dendritic-cell progeny serve three main functions in the
immune system.
These are phagocytosis, antigen presentation, and cytokine production.
Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction
of this material.
Monocytes can perform phagocytosis using intermediary proteins such as antibodies or complement
that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition
receptors that recognize pathogens.
Monocytes are also capable of killing infected host cells via antibody-dependent cell-mediated
cytotoxicity.
Vacuolization may be present in a cell that has recently phagocytized foreign matter.
Microbial fragments that remain after such digestion can serve as antigens.
The fragments can be incorporated into MHC molecules and then trafficked to the cell
surface of monocytes.
This process is called antigen presentation and it leads to activation of T lymphocytes,
which then mount a specific immune response against the antigen.
Other microbial products can directly activate monocytes and this leads to production of
pro-inflammatory and, with some delay, of anti-inflammatory cytokines.
Typical cytokines produced by monocytes are TNF, IL-1, and IL-12.
Monocyte subpopulations There are at least three types of monocytes
in human blood: a) the classical monocyte is characterized
by high level expression of the CD14 cell surface receptor
b) the non-classical monocyte shows low level expression of CD14 and additional co-expression
of the CD16 receptor.
c) the intermediate monocyte with high level expression of CD14 and low level expression
of CD16.
Ghattas et al. state that the "intermediate" monocyte population is likely to be a unique
subpopulation of monocytes, as opposed to a developmental step, due to their comparatively
high expression of surface receptors involved in reparative processes(including vascular
endothelial growth factors type 1 and 2, CXCR4, and Tie-2) as well as evidence that the "intermediate"
subset is specifically enriched in the bone marrow.
After stimulation with microbial products the CD14+CD16++ monocytes produce high amounts
of pro-inflammatory cytokines like tumor necrosis factor and interleukin-12.
Said et al. showed that activated monocytes express high levels of PD-1 which might explain
the higher expression of PD-1 in CD14+CD16++ monocytes as compared to CD14++CD16- monocytes.
Triggering monocytes-expressed PD-1 by its ligand PD-L1 induces IL-10 production which
activates CD4 Th2 cells and inhibits CD4 Th1 cell function.
Diagnostic use
A monocyte count is part of a complete blood count and is expressed either as a ratio of
monocytes to the total number of white blood cells counted, or by absolute numbers.
Both may be useful in determining or refuting a possible diagnosis.
Monocytosis Monocytosis is the state of excess monocytes
in the peripheral blood.
It may be indicative of various disease states.
Examples of processes that can increase a monocyte count include:
chronic inflammation stress response
Cushing's syndrome immune-mediated disease
pyogranulomatous disease necrosis
red blood cell regeneration Viral Fever
sarcoidosis A high count of CD14+CD16++ monocytes is found
in severe infection and a very low count of these cells is found after therapy with immuno-suppressive
glucocorticoids.
Monocytopenia Monocytopenia is a form of leukopenia associated
with a deficiency of monocytes.
Dendritic cells Monocytes can be used to generate dendritic
cells in vitro by adding cytokines like Granulocyte Monocyte Colony Stimulating Factor and IL-4.
Blood content
See also Agranulocyte
Complete blood count Hematopoiesis
Lymphocyte Neutrophil granulocyte
Phagocyte White blood cell
References
External links Histology image: 01702ooa - Histology Learning
System at Boston University Human Monocytes - Prof. Dr. Ziegler-Heitbrock
Circulation of Body Fluids