Johnson & Johnson (JNJ 1.49%) just took a big step into the field of immunology with a $6.5 billion deal for Momenta Pharmaceuticals (MNTA). On Tuesday, the giant healthcare conglomerate announced a definitive agreement to acquire the biotech for $52.50 per share. That works out to a whopping 70% premium over the Momenta's closing price Monday.

One of the key assets that inspired such a generous offer is nipocalimab, a potential first-in-class antibody that targets the neonatal Fc receptor, FcRn. But geography and talent were also major factors. The healthcare conglomerate is eager to expand its presence in Cambridge, Massachusetts, the world's leading hub for biopharmaceutical innovation and the location of Momenta's headquarters.

Scientist counting out cash money.

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Inhibiting FcRn with nipocalimab appears to reduce the concentrations of circulating immunoglobulin G antibodies thought to be responsible for damaging the nerve endings of patients with generalized myasthenia gravis and related autoimmune disorders. During a 68-patient dose-determination trial, 52% of those treated with the FcRn inhibitor reported significant symptom reductions compared to 15% of those who received a placebo.

Myasthenia gravis is one of many rare disorders driven by nerve-damaging autoantibodies -- antibodies that impede that healthy function of the body rather than attacking invaders. Nipocalimab is currently being tested in a phase 3 trial as a treatment for warm autoimmune hemolytic anemia patients, and Johnson & Johnson thinks there could many more indications in the future for autoantibody-targeting treatments. According to the company's estimate, roughly 195 million people worldwide suffer from a rare disease driven by autoantibodies.