T-cell receptors (TCRs) are membrane-bound proteins on T lymphocytes, recognizing antigenic peptides presented by MHC molecules, initiating the immune response. They have diverse V domains for antigen recognition, C domains for structural support, associate with CD3 complex for signaling, and αβ or γδ chains. TCRs enable diverse antigen recognition critical for T cell functions.

Checkpoint inhibitors are a type of cancer immunotherapy that block inhibitory pathways in the immune system, enhancing its ability to recognize and attack cancer cells. Targeting interactions between inhibitory molecules on T cells and their ligands, such as CTLA-4 and PD-1, these inhibitors aim to reinvigorate antitumor immune responses, though not all patients respond. Ongoing research seeks to improve outcomes and identify predictive biomarkers.

Costimulatory signals determine CD4+ T cell activation or suppression. Key pairs like CD28:B7-1/B7-2 and CD40:CD40L provide activation, while CTLA-4:B7-1/B7-2 and PD-1:PD-L1/PD-L2 deliver suppression. Factors like expression levels and microenvironment influence their effects. Intracellular pathways also regulate T cell fate. This balance prevents immune dysregulation and autoimmunity.