Methylprednisolone binds to and activates intracellular glucocorticoid receptors. Activated glucocorticoid receptors bind to promoter regions of DNA (which may activate or suppress transcription). Unbound glucocorticoids cross cell membranes and bind with high affinity to specific cytoplasmic receptors, modifying transcription and protein synthesis. By this mechanism, glucocorticoids can inhibit leukocyte infiltration at the site of inflammation, interfere with mediators of inflammatory response, and suppress humoral immune responses. The antiinflammatory actions of corticosteroids are thought to involve phospholipase A2 inhibitory proteins, lipocortins, which control the biosynthesis of potent mediators of inflammation such as prostaglandins and leukotrienes.