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Keywords:

  • immunodeficiency;
  • T follicular helper cells;
  • T-dependent B cell activation;
  • T–B cell interactions;
  • humoral immunity;
  • XLP

Regulated interactions between cells of the immune system facilitate the generation of successful immune responses, thereby enabling efficient neutralization and clearance of pathogens and the establishment of both cell- and humoral-mediated immunological memory. The corollary of this is that impediments to efficient cell–cell interactions, normally necessary for differentiation and effector functions of immune cells, underly the clinical features and disease pathogenesis of primary immunodeficiencies. In affected individuals, these defects manifest as impaired long-term humoral immunity and susceptibility to infection by specific pathogens. In this review, we discuss the importance of, and requirements for, effective interactions between B cells and T cells during the formation of CD4+ T follicular helper cells and the elicitation of cytotoxic function of virus-specific CD8+ T cells, as well as how these processes are abrogated in primary immunodeficiencies due to loss-of-function mutations in defined genes.