The next reaction pattern to look for in the dermis is the Granulomatous pattern. Granulomas are collections of histiocytic (macrophage) cells and lymphocytes (T cells) surrounding a central focus of infection or altered collagen or elastin or ocassionally other foreign bodies. The lymphocytes represent the T cell response of the acquired immune system trying to localise or remove a foreign antigen partly taken up by the macrophages. Nontheless the commonest cause of granulomas in the skin is the body's response to ruptured cysts or infected follicles or foreign body reactions to gout crystals or retained sutures or suture material.
Certain features of granulomas suggest certain diseases but there is significant overlap and some are not true to type. eg Palisading granulomas have the lymphohistiocytic cells surrounding degenerated collagen, elastin or mucin or fibrin. The main examples are Granuloma annulare, Necrobiosis lipoidica and Rheumatoid nodule. Because these are all dermalhistological changes they present as smoothe surfaced plaques or nodules generally skin coloured.
Caseating granuloma is characteristic of TB with necrosis developing in the centre of the granuloma.
Tuberculoid granulomas are described as naked granulomas because they are mainly made up of macrophages with very few surrounding lymphocytes. The main conditions showing this histological feature are Sarcoidosis, non caseating forms of TB and Leprosy where there is a highly developed immune response and few remaining organisms.
