DISCLOSED HEREIN ARE ISOLATED AND PURIFIED STAPHYLOCOCCUS AUREUS BI-COMPONENT LEUCOCIDIN, REFERRED TO HEREIN AS LUKAB, AND ITS COMPONENTS LUKA AND LUKB, ANTIBODIES SPECIFIC TO LUKA, ANTIBODIES SPECIFIC TO LUKB, THERAPEUTIC COMPOSITIONS CONTAINING LUKA AND/OR LUKB, OR ANTI-LUKA AND/OR ANTI-LUKB ANTIBODIES, USES OF THE COMPOSITIONS TO TREAT ACUTE INFLAMMATORY CONDITIONS OR S. AUREUS INFECTION, METHODS FOR IDENTIFYING INHIBITORS OF LUKAB-MEDIATED CYTOTOXICITY OF HUMAN PHAGOCYTES, AND METHODS FOR USING LUKAB AS A MARKER TO PREDICT SEVERITY OF S. AUREUS INFECTION. FIGURE 1-1