IMMUNOLOGY

Detailed explanation-1: -Adaptive immune responses depend on the ability to recognize and eliminate recurrent pathogens, resulting in the generation of memory lymphocytes. The capacity of memory T cells to rapidly mobilize and initiate a potent recall response enhances protective immunity against previously encountered pathogens.

Detailed explanation-2: -T cells are a part of the immune system that focuses on specific foreign particles. Rather than generically attack any antigens, T cells circulate until they encounter their specific antigen. As such, T cells play a critical part in immunity to foreign substances.

Detailed explanation-3: -T-cell memory is a critical component of immune responses to intracellular pathogens. Following the antigen-driven expansion and the death of effector cells after antigen clearance, some of the remaining T cells differentiate into memory T cells of two different types: central memory and effector memory T cells.

Detailed explanation-4: -In addition to “fighting off” these pathogens, active immunity is important because it lasts a long time in the form of immunologic memory. Immunologic memory consists of B and T cells that can recognize a particular pathogen (see “Adaptive immune system").