Immigrants founded or cofounded almost half of 50 top venture-backed companies in the United States, a new study shows, underscoring some of the high stakes in potential immigration reform.

He was also an illegal immigrant. So when his younger brother volunteered to donate a kidney to restore him to normal life, they encountered a health care paradox: the government would pay for a lifetime of dialysis, costing $75,000 a year, but not for the $100,000 transplant that would make it unnecessary.