Granuloma

A gathering of activated macrophages that are grouped together into a nodule, and surrounded by lymphocytes. The macrophages often develop an epithelial-like character, thus aquiring the name epitheloid cells.

Epitheloid Granuloma
A sharply circumscribed nodule that mainly consists of epitheloid cells(modified macrophages).
 * Tuberculous granuloma.
 * Sarcoidal granuloma.

Histiocytic Granuloma
An ill-defined nodule that consists of phagocytic histiocytes(tissue macrophages).
 * Foreign-body granuloma.
 * Rheumatoid granuloma.
 * Rheumatic granuloma.

Sarcoid Granuloma
Small granulomas that mostly consists of epitheloid cells. No necrotizing center, but fibrosis may be present. The outer layer consists mostly of collagen.