Development and Structure of Histiocytes
- Histiocytes are derived from bone marrow through multiplication from a stem cell.
- Derived cells migrate from bone marrow to the blood as monocytes.
- Monocytes circulate through the body and enter various organs.
- In organs, monocytes differentiate into histiocytes.
- Histiocytes have common histological and immunophenotypical characteristics.
- Their cytoplasm is eosinophilic and contains lysosomes.
- Histiocytes bear membrane receptors for opsonins like IgG and complement fragment C3b.
- They express leucocyte common antigens (LCAs) CD45, CD14, CD33, and CD4.
- CD4 is also expressed by T helper cells.

Macrophages and Dendritic Cells
- Macrophages and dendritic cells are part of the mononuclear phagocytic system (MPS).
- Macrophages specialize in phagocytosis.
- Dendritic cells specialize in antigen presentation.
- Both cells are derived from common bone marrow precursor cells.
- Differentiation into histiocytes is influenced by tissue location and growth factors.

Langerhans Cells
- Langerhans cells differentiate from a subset of histiocytes.
- Maturation occurs in squamous epithelium, lymph nodes, spleen, and bronchiolar epithelium.
- Langerhans cells are antigen-presenting cells.
- They express CD1a and S-100.
- Their cytoplasm contains Birbeck granules.

Clinical Significance of Histiocytoses
- Histiocytoses are neoplasias where the proliferative cell is the histiocyte.
- The most common histiocyte disorders are Langerhans cell histiocytosis and haemophagocytic lymphohistiocytosis.
- Langerhans cell histiocytosis involves skin, lymph nodes, spleen, and bronchiolar epithelium.
- Haemophagocytic lymphohistiocytosis affects immune cells and can be life-threatening.
- Accurate diagnosis and treatment are crucial for managing histiocytoses.

Miscellaneous
- Histiocytes are part of the mononuclear phagocytic system (MPS).
- Histiocytes express leucocyte common antigens (LCAs) CD45, CD14, CD33, and CD4.
- CD4 is also expressed by T helper cells.