IMMUNOLOGY

Detailed explanation-1: -Memory B cells can survive for decades, which gives them the capacity to respond to multiple exposures to the same antigen.

Detailed explanation-2: -The lifespan of a human memory T cell is of the order of 30–160 days [12, 13, 15, 16], in contrast to the typical half-life of human T cell memory of 8–15 years [1, 4, 5]. Longevity thus does not seem to be an intrinsic characteristic of circulating memory T cells.

Detailed explanation-3: -The memory B cells are activated by the variant pathogen to differentiate into long-lived plasma cells or to re-enter the geminal centres (GCs) to replenish the memory B cell pool.

Detailed explanation-4: -During a primary immune response to new pathogens or antigens, naive T cells are activated, and undergo proliferative expansion and differentiation into effector cells. Although most effector cells die after antigen clearance, some persist as memory T cells.

Detailed explanation-5: -Memory B cells can survive in secondary lymphoid organs in the absence of antigen and mediate secondary immune response on restimulation. Moreover, there is evidence that memory B cells also maintain serologic memory by undergoing continuous polyclonal activation.