A histiocyte is a vertebrate cell that is part of the mononuclear phagocyte system (also known as the reticuloendothelial system or lymphoreticular system). The mononuclear phagocytic system is part of the organism's immune system. The histiocyte is a tissue macrophage
Development
Histiocytes are derived from the
bone marrow by multiplication from a
stem cell. The derived cells migrate from the bone marrow to the
blood as
monocytes. They circulate through the body and enter various organs, where they undergo differentiation into histiocytes, which are part of the mononuclear phagocytic system (MPS).
However, the term histiocyte has been used for multiple purposes in the past, and some cells called "histocytes" do not appear to derive from monocytic-macrophage lines.
Some sources consider Langerhans cell derivatives to be histiocytes.
Structure
Histiocytes have common
histology and immunophenotypical characteristics (demonstrated by
immunostaining). Their
cytoplasm is
eosinophilic and contains variable amounts of
. They bear membrane receptors for
, such as
Immunoglobulin G and the fragment C3b of complement. They express LCAs (leucocyte common antigens) CD45, CD14, CD33, and CD4 (also expressed by T helper cells).
Macrophages and dendritic cells
These histiocytes are part of the immune system by way of two distinct functions:
phagocytosis and antigen presentation. Phagocytosis is the main process of
and
antigen presentation the main property of
(so called because of their star-like cytoplasmic processes).
and are derived from common bone marrow precursor cells that have undergone different differentiation (as histiocytes) under the influence of various environmental (tissue location) and growth factors such as GM-CSF, TNF and IL-4. The various categories of histiocytes are distinguishable by their morphology, phenotype, and size.
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Macrophages are highly variable in size and morphology, their cytoplasm contains numerous acid phosphatase laden lysosomes – in relation to their specialised phagocytic function. They express CD68.
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Dendritic cells have an indented (bean-shaped) nucleus and cytoplasm with thin processes (dendritic). Their main activity is antigen presentation; they express , CD1c, and Class II Human leukocyte antigens.
Langerhans cells
A subset of cells differentiates into
Langerhans cells; this maturation occurs in the squamous epithelium,
lymph nodes,
spleen, and bronchiolar epithelium. Langerhans cells are antigen-presenting cells but have undergone further differentiation. Skin Langerhans cells express CD1a, as do cortical
thymocytes (cells of the cortex of the
thymus gland). They also express S-100, and their cytoplasm contains tennis-racket like ultra-structural inclusions called
Birbeck granules.
Clinical significance
Histiocytosis describe
neoplasias wherein the proliferative cell is the histiocyte. The most common histiocyte disorders are Langerhans' cell histiocytosis and haemophagocytic lymphohistiocytosis.
See also
