Figure 1.

Presentation of antigen-derived peptides to T-cells. The activation of CD4 T-cells in an antigen-specific manner requires a process called antigen presentation and performed by specialized immune cells named antigen-presenting cells (APCs). APCs harbor unique abilities to capture extracellular antigens, to process (i.e., cleave) these antigens into short peptides and to expose at the outer surface of their cellular membrane the peptides generated by antigen processing. There, the exposed peptides are “recognized” by T-cells. Importantly, the recognition of antigen-derived peptides by T-cells also requires that such peptides physically associate with molecules of the major histocompatibility complex class II (MHC class II) that are expressed by APCs. The antigen-specific activation of a CD4 T-cell engages only if the TCR (T-cell receptor) expressed at its cell surface binds with a high affinity the molecular complex formed by: (i) a peptide derived from the targeted antigen and (ii) MHC class II molecules into which the antigen-derived peptide is loaded.