When B cells and T cells are activated, some become memory cells. Throughout the lifetime of an animal, these memory cells form a database of effective B and T lymphocytes. Upon interaction with a previously-encountered antigen, the appropriate memory cells are selected and activated. In this manner, the second and subsequent exposures to an antigen produce a stronger and faster immune response. This is “adaptive” because the body’s immune system prepares itself for future challenges, which can stop an infection by the same pathogen before it can even cause symptoms.
Antibody: An antibody is made up of two heavy chains and two light chains. The unique variable region allows an antibody to recognize its matching antigen
Immunological memory can either be in the form of passive short-term memory or active long-term memory. Passive memory is usually short-term, lasting between a few days and several months, and is particularly important for newborn infants, who are given passive memory from maternal antibodies and immune cells before birth. Active immunity is generally long-term and can be acquired by infection followed by B cells and T cells activation, or artificially acquired by vaccines in a process called immunization.
The memory system does have a few flaws. Pathogens that undergo mutation often have different antigens than those known by memory B and T cells, meaning that different strains of the same pathogen can avoid the memory-enhanced immune response. Additionally, the memory cell function enables the development of hypersensitivity disorders, such as allergies and many chronic diseases (like multiple sclerosis or myasthenia gravis). In these cases, memory cells form for an antigen that elicits an immune response without actually being caused by a pathogen, which leads to immune system mediated-damage to the body from mast cell, antibody, or T-cell mediated activities and inflammation.