histiocytic

Histiocytic is a term used to describe cells or substances related to histiocytes, which are tissue-resident macrophages derived from circulating monocytes. Histiocytes are distributed throughout body tissues as part of the immune system and act as phagocytic and antigen-presenting cells.

In normal physiology, histiocytes perform phagocytosis of pathogens and debris, clear apoptotic cells, present antigens to T cells, and secrete cytokines that coordinate inflammation and tissue repair. They are found in the reticuloendothelial system, including lymph nodes, spleen, liver, and skin, as well as in many organs.

Disorders involving histiocytes range from localized proliferations to systemic syndromes. Langerhans cell histiocytosis (LCH) is a clonal disorder of Langerhans cells, a specialized dendritic cell lineage, often expressing CD1a and langerin (CD207) and sometimes S-100. Non-Langerhans histiocytoses involve macrophage- or dendritic cell lineages lacking those markers.

Histiocytic disorders include hemophagocytic lymphohistiocytosis (HLH), a hyperinflammatory syndrome driven by macrophage activation; Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy), which features S-100 positive histiocytes with emperipolesis; Erdheim-Chester disease, a non-Langerhans histiocytosis with foamy histiocytes infiltrating bones and other tissues; and juvenile xanthogranuloma, typically presenting in infancy or childhood with nodules.

Diagnosis relies on tissue biopsy with immunohistochemistry and clinical context. Common histiocytic markers include CD68 and CD163; LCH cells express CD1a and langerin; Rosai-Dorfman histiocytes are S-100 positive and show emperipolesis. Treatment varies by disease and may involve observation, corticosteroids, chemotherapy, immunotherapy, or targeted therapies in select cases.