Figure 82

Pathogenesis of granulomatous lesions. Mononuclear cell infiltration is the initial step in the sequence of events that leads to granuloma formation. Recruited macrophages then differentiate into epithelioid and multinucleated giant cells. Activated lymphocytes are interspersed in the evolving lesion and come to form a rim around the granulomas. In time, fibroblasts, mast cells, and collagen fibers begin to encapsulate the mature sarcoid granuloma. Cultured granulomatous homogenates exhibit 1 a-hydroxylase activity and are capable of converting 25-hydroxy-vitamin D3 to its active 1,25-dihydroxylated form, calcitriol. This capacity resides in the infiltrating macrophages and is not unique to sarcoidosis but a feature of most other granulomatous disorders. Although lacking in specificity to be of diagnostic merit, radioactive gallium scans can be used as noninvasive methods of assessing the activity of sar-coid granulomas. The uptake of radioactive gallium by the macrophages and lymphocytes reflects the activity of the infiltrating cells in affected organs.

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