KRISTOFFER TRIPPLAAR/SIPA/AP

AstraZeneca on Saturday said it was acquiring Alexion Pharmaceuticals for $39 billion, marking a significant expansion into the treatment of rare diseases of the immune system.

The deal — a mix of cash and stock — values Alexion at $175 per share, a 45% premium to Friday’s closing stock price.

Image: KRISTOFFER TRIPPLAAR/SIPA/AP