In biology, a memory cell refers to one of a number of types of cells that make up part of the immune system. Memory cells are a vital part of the system that defends the body against pathogens such as bacteria or viruses that cause disease and infection. The memory cells are also referred to as white blood cells or lymphocytes. There are two main types of memory cells, called memory T cells and memory B cells. T cells activate the immune system and directly attack pathogens, while B cells produce substances called antibodies, which can disable or kill pathogens.

A memory cell starts its life in the bone marrow, where lymphocytes are made. It is then transported around the body in lymph, a clear liquid that, among other functions, transports lymphocytes to regions of infection. Lymph is transported around the body via the lymphatic system, a network of vessels and tissues throughout the body.

Memory cell function is characterized by the memory present in acquired immunity. Once a memory cell, either a T cell or a B cell, has been exposed to a specific pathogen, it will react much more rapidly when it encounters the same pathogen in the future. This is the reason that some diseases can normally only be caught once by a person. If a person suffers from an infection such as measles, his or her memory cells "learn" how to get rid of the virus that causes the disease. Once the measles virus has been successfully fought off once, then any future infections with measles viruses will normally be repulsed without the person becoming ill.

Another example of how memory cells work in the immune system includes vaccination. A vaccine actually contains the pathogen that causes the disease that the vaccine is designed to prevent. The pathogen in the vaccine is either weakened or dead, so that it is not usually strong enough to make the recipient of the vaccine ill. However, the pathogen in its weakened form still stimulates the immune system, allowing the memory cells to "learn" how to fight off that specific pathogen. In the future, if the person is exposed to the pathogen in its virulent form, he or she is much more likely to be able to recover from the infection quickly, either not becoming ill, or only suffering a mild version of the disease.