May 22 (Reuters) - Biogen said on Wednesday it

had agreed to buy privately held Human Immunology Biosciences

for up to $1.8 billion, gaining access to targeted therapies for

patients with severe immune-related diseases.

The deal consists of $1.15 billion in upfront and up to $650

million in potential milestone payments if Human Immunology's

lead product felzartamab, a monoclonal antibody, achieves

certain development milestones.

The purchase comes as Biogen's pushes to move beyond

treatments for neurological disorders and boost its portfolio of

rare disease drugs.

The companies expect to close the deal in the third quarter.