Abstract
As a major arm of the cellular immune response, CD4(+) T cells are important in the control and clearance of infections. Primarily described as helpers, CD4(+) T cells play an integral role in the development and activation of B cells and CD8(+) T cells. CD4(+) T cells are incredibly heterogeneous, and can be divided into six main lineages based on distinct profiles, namely T helper 1, 2, 17 and 22 (Th1, Th2, Th17, Th22), regulatory T cells (Treg) and T follicular helper cells (Tfh). Recent advances in structural biology have allowed for a detailed characterisation of the molecular mechanisms that drive CD4(+) T cell recognition. In this review, we discuss the defining features of the main human CD4(+) T cell lineages and their role in immunity, as well as their structural characteristics underlying their detection of pathogens.