While a lot of the monoclonal antibodies come from humans, some still come from animals, including mice, Casadevall says. In fact, mice don't need to even get the disease in order to mount an immune response, which includes the manufacture of antibodies against it, he adds.
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Rheumatic diseases are routinely treated with monoclonals, and a monoclonal that neutralizes IgE is used in severe asthma.
"Infectious disease is where antibody treatments were pioneered," Casadevall said. "But now there are dozens of monoclonals licensed for use in cancer. It's had an enormous impact on cancer over the last 15 years since it was introduced. It's been effective in the treatment of breast cancer and colon cancer."
Why the two Americans were chosen for this treatment, while many others are sick with Ebola in Africa, was unclear.
"While the FDA cannot comment on the development of specific medical products, it's important to note that every FDA regulatory decision is based on a risk-benefit assessment that includes the context of use for the product and the patient population being studied," said Stephanie Yao, a spokesperson for the FDA.
—By Linda Carroll, NBC News