Memory cells are a type of white blood cell that play a crucial role in the immune system. They are formed after the body encounters a specific pathogen, such as a virus or bacteria. Once created, memory cells remain in the body for a long time, allowing the immune system to recognize and respond more quickly if the same pathogen is encountered again in the future. These cells are essential for long-lasting immunity, which is the basis for how vaccines work. When a vaccine introduces a harmless part of a pathogen, it stimulates the production of memory cells. This prepares the immune system to fight off the actual pathogen if it is encountered later, providing protection against diseases.