In this article we present the role and discuss the significance of monocytes
in both innate and adaptive immunity. Monocytes are the largest white blood
cells of convoluted, bilobed nuclei. Growth factors and cytokines determine
the monocyte subtype. The monocyte functional and phenotype heterogeneity
have been recognized and in the recent years there have been identified their
major populations in humans; classical (CD14+CD16−), non-classical (CD14dim
CD16+), and intermediate (CD14+CD16+). Each of these subpopulations
is distinguished by the expression of distinct surface markers and by their
functions in homeostasis and disease. Monocytes develop in bone marrow
and travel to tissues and organs in where they become macrophages or
dendritic cells. Macrophages are effector cells of the innate immune system
that phagocytose bacteria and secrete both pro-inflammatory and antimicrobial
mediators. In addition, macrophages play an important role in eliminating
diseased and damaged cells through their programmed cell death. Generally,
macrophages ingest and degrade dead cells, debris, tumor cells, and foreign
materials. Dendritic cells represent a heterogeneous family of immune cells that
link innate and adaptive immunity. Their main function is to process antigen
material and present it on the cell surface to the T cells of the immune system.
Monocytes differentiated into dendritic cells play an important role in innate
and adaptive immunity, due to their microbicidal potential, capacity to stimulate
CD4+
and CD8+
T-cell responses and ability to regulate immunoglobulins
production by B cells.