Memory cells are specialized types of white blood cells that play a crucial role in the immune system. They are formed after the body encounters a pathogen, such as a virus or bacteria. Once activated, these cells remember the specific characteristics of the pathogen, allowing for a faster and more effective response if the body encounters the same pathogen again in the future. There are two main types of memory cells: B cells and T cells. B cells produce antibodies that target specific pathogens, while T cells help destroy infected cells and coordinate the immune response. Together, these memory cells provide long-lasting immunity, which is the basis for how vaccines work.