- Granulomas are a unique morphological pattern of inflammation which occur in response to particular microbial infections, foreign bodies, or in certain autoimmune contexts. Granulomatous inflammation may have evolved for the purpose of physically quarantining infectious processes which cannot otherwise be resolved by the immune response. Consequently, the morphological architecture of the granuloma appears to be designed to physically isolate infectious processes from the rest of the body.
- Unless surrounding a large, irregularly-shaped foreign body, most granulomas appear as roughly spherical structures with several layers. At the center of the granuloma lies the quarantined material which is surrounded by several layers designed to isolate it from the outer normal tissue.
- At the center of the granuloma lies the isolated material. When an infectious process is the object of quarantine, the central area may appear as a region of Caseous Necrosis, representing dead cellular and microbial debris. In cases where granulomas appear due to cryptic autoimmune processes, no infectious process is at work and thus the center does not display caseation, resulting in a Non-caseating Granuloma. Granulomas can also form around large, indigestible foreign bodies such as thorns or accumulations of inorganic dust.
- Central Layer
- Surrounding the central area of quarantine lies a tight ring of macrophages which possess a epithelial-like morphology. In some cases several macrophages may fuse to form syncytial, multi-nucleated "Giant Cell".
- Outer Layer
- Surrounding the macrophage layer lies a ring of lymphocytes, primarily of the CD4+ T-cell subtype. These T-cells appear to secrete large amounts of Interferon-gamma which stimulates the phagocytic and anti-microbial capacity of nearby macrophages as well as helps coordinate and maintain the presence of the granuloma architecture. Over time, this outer ring of lymphocytes may itself become surrounded by fibroblasts which elaborate a thick ring of fibrosis, likely acting as a near-permanent seal of the isolated material.