What do memory cells do upon re-encountering an antigen?

Memory cells play a critical role in the immune system by providing a faster and more robust response upon re-encountering an antigen. When the immune system first encounters a pathogen, it generates a primary response that includes the activation of B cells and T cells. Some of these cells differentiate into memory cells, which persist in the body long after the initial infection has been cleared.

Upon subsequent exposures to the same antigen, these memory cells are able to recognize it quickly due to their previous experience. This recognition enables them to respond much more rapidly compared to the initial response. Memory B cells, for instance, can rapidly produce large amounts of antibodies, while memory T cells can quickly proliferate and activate to eliminate the pathogen.

The strength and speed of this secondary immune response result in a greater level of protection against the pathogen, often preventing illness or significantly reducing the severity of the disease. This principle is the basis for why vaccines, which help induce the formation of memory cells, are effective in providing immunity.