At 48m35s into this video, Fauci answers a question about antibody longevity in COVID as saying:

"There are some people where antibodies last only a short period of time. What we need to know is what that means. When antibodies are no longer detectable, are there other aspects of the immune response, like T-Cell or Cellular Immunity, that could still protect them from re-infection."

https://www.youtube.com/watch?v=njMfCX46Ej4

I only know high school biology, but I thought presence of antibodies was the sole factor in an individual's immunity from a virus, because they are "memory cells" for detecting pathogens.

Is Fauci really saying people can still be immune even if their antibodies have gone?