Immune cells called T cells can play a key role in maintaining protection against Covid-19 hospitalizations and deaths when new variants arise, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

Throughout the Covid-19 pandemic, immunity has often been measured by the presence of antibodies in the blood. Antibodies are proteins made by the immune system to help fight infections. But immune systems are much more than just antibodies. They involve many players, including B cells and T cells, which are involved in the effort to fight off the coronavirus or any other pathogen.

During a White House briefing on Friday, Fauci said:

"The fundamental basic message is that there are two limbs of the response. The B cell limb, which provides the antibodies that we measure when we show you neutralizing titers. Those B cell responses can give antibodies but also memory, namely, when they see an antigen again."

Fauci added that "the other limb of the immune response is the T cell limb," which, in some respects, helps the B cells make antibodies but also has a function of its own, including immune memory function. 

Antibodies — made by B cells with help from T cells — primarily prevent initial infection, Fauci said. T cells generally prevent the progression of an infection.