This story is republished from STAT, the health and medicine news site that’s a partner to the Globe. Sign up for STAT’s free Morning Rounds newsletter here. Biogen said Tuesday that it would buy Apellis Pharmaceuticals for an upfront payment of around $5.6 billion, expanding its portfolio with a set of approved immunology drugs. Cambridge-based Biogen will acquire Apellis, headquartered in Waltham, for $41 per share, more than double the latter’s closing price on Monday.