There are many different granulomatous diseases, from Crohn's disease to tuberculosis. The cells that make up a granuloma are from the immune system. The immune system is the body's defense against disease and illness. Its major players are the macrophages and leukocytes, cells that originate in the bone marrow and travel through the lymphatic vessels to different areas of the body. Macrophages are cells that attack foreign microbes by binding to and engulfing them.
Macrophages secrete assorted biochemicals that affect the behavior of the surrounding cells.
One of the types of biochemicals that they secrete, for example, are the cytokines, which cause inflammation. This is why an infected area becomes inflamed. They then start to secrete antibodies to fight the attacker. The two most common types of lymphocytes are T cells and B cells. The granuloma is a cluster of a variety of cells that forms a discrete nodule.