What do memory cells do?

During a subsequent encounter, the memory cell recognizes the pathogen and mounts a quick and effective response against it. This response, called the secondary immune response, is faster and stronger than the primary response. 

Memory cells do not respond immediately on their first exposure to a new pathogen. However when it encounters the pathogen on a subsequent encounter, it facilitates a rapid and strong secondary response. Memory cells can recognize a pathogen even several years after the first exposure. 

Memory cells may be memory T cells or memory B cells, both of which are clones of the parent T cell and B cell respectively. Both respond differently to a subsequent invasion. On recognizing a repeat pathogen, memory T cells replicate quickly and respond to the infection more strongly. On the other hand, on recognizing a repeat pathogen, memory B cells produce a more robust antibody-mediated immune response by pumping out thousands of neutralizing antibodies per second.