In the setting of reperfusion therapy in an acute myocardial infarction using primary percutaneous intervention (PCI), the body's own inflammatory system involving the complement cascade may be harmful. This study will test the safety and efficacy of a novel complement inhibitor, pexelizumab to reduce mortality at 30 days.

Inclusion Criteria:

• Cardiac symptoms for at least 20 minutes within past 6 hours;
• Will undergo primary PCI;
• Has ECG evidence of acute high risk ST elevation myocardial infarction;
• Willing and able to be followed for at least 12 months.

Exclusion Criteria:

• Isolated low risk inferior wall myocardial infarction;
• Received fibrinolytic therapy;
• History of complement deficiency;
• Suspected neisserial infection;
• Participating in other investigational study;
• Pregnancy;
• Previous enrollment.