Effector cells and memory cells are two types of immune system cells. There are a few significant differences between the two:

Effector cells:

• Are activated lymphocytes that play a critical role in defending the body from foreign pathogens 
• Are relatively short-lived
• Their main function is to fight off new pathogens that invade the body  

Memory cells:

• Are long-lived lymphocytes that are can recognize previously encountered antigens long after the initial exposure 
• Are relatively long-lived with a life span that can last throughout the organism’s life
• Their main function is to recognize a previously encountered antigen and mount a swift and robust immune response