Scientists have long assumed that Ebola's infamously high mortality rate was due to an ability to knock out the body's immune system and cause certain white blood cells to self-destruct, among other effects.

However, new research published Monday in the journal PNAS suggests that the human immune system doesn't give up that easily when confronted with Ebola virus disease.

In tests performed on the four Ebola patients treated at Atlanta's Emory University Hospital, doctors discovered that each of the patients' immune systems had mounted a surprisingly strong counterattack.

"We found a striking activation of both B and T cells in all four patients," wrote lead study author Anita McElroy, an infectious disease researcher at Emory.

McElroy and her colleagues found that immature B cells, or plasmablasts, that were actively secreting antibody accounted for 50 percent of all B cells in infected individuals, compared with less than 1 percent in healthy individuals.

Likewise, the frequency of activated CD4 T cells - white blood cells that help direct immune cell response - ranged from 5 percent to 30 percent in the patients, compared with 1 percent to 2 percent in healthy individuals.

But the most profound response was in the patient's CD8 T cells, or "killer T cells," researchers said. At least half of them showed signs that they were activated for battle.

"Taken together, these results suggest that all four patients developed robust immune responses during the acute phase of Ebola virus infection, a finding that would not have been predicted based on our current assumptions about the highly immunosuppressive nature of Ebola virus," authors wrote.

All four patients recovered. Among them were aid workers Kent Brantly and Nancy Writebol, who both received doses of the experimental drug ZMapp.