Granulomatous diseases are a large group of diseases which share in the process of forming granulomas. These are immune reactions against something foreign which take a specific globular shape of cells. Examples of such conditions include tuberculosis, sarcoidosis, leprosy and cat-scratch disease.
The mechanism by which such diseases can cause leukopenia is quite complex, and although some rare diseases exist where both conditions are independent, such as chronic granulomatous disease, the main mechanism of action in most cases is through the invasion of the bone marrow by such granulomas. In such a case, they act in a similar way to cancer.