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The test, using an antigen called leishmanin, is similar to a skin test for tuberculosis -- a positive response means a person has been exposed to the parasite and has cellular immunity that prevents further clinical symptoms.

That unseen mechanism was what scientists call cellular immunity, and its foot soldiers are T cells.

If a lab test were able to measure cellular immunity as easily as antibodies, “that would spread like wildfire, because everyone is searching for one,” Meissner said.

There’s also encouraging evidence that this cellular immunity is long-lasting.

“It’s a way — the only scalable way we have now — to quantify cellular immunity,” said Ernesto Guccione, a Mount Sinai immunology researcher who helped design the test and has a patent pending.