Activation of naive, memory and effector T cells.

An increased understanding of the types of T-cell subsets that exist in vivo, their relationships to one another, and how to identify and isolate them or effect their generation, has led to a comprehensive view of the antigen-presenting cells (APCs) which may be active and regulatory during the course of an immune response. Recent studies show that naive T cells only respond efficiently to dendritic cells and activated B cells whereas memory and effector cells respond to all APC types to some extent, including resting B cells. High level co-stimulatory molecule expression largely explains why APCs such as dendritic cells are far more effective stimulators than resting B cells. The available data, therefore, suggest that the requirement for co-stimulation, and hence capacity to respond to various APCs, is largely a function of the differentiation state of the T cell, and that previous encounter with antigen fundamentally increases the ability of T cells to subsequently respond to antigen rechallenge.