Final answer:
The CD4 receptor on Th cells recognizes peptide antigens bound to Major Histocompatibility Complex II proteins, which are presented by antigen-presenting cells, leading to the activation of Th cells.
Step-by-step explanation:
The CD4 receptor on T helper (Th) cells recognizes peptide antigens when they are bound to Major Histocompatibility Complex (MHC) II proteins. T cells, including Th cells, have a T cell receptor (TCR) that is involved in the recognition of processed foreign epitopes presented alongside MHC molecules. Specifically, the CD4+ Th cells interact with antigens that are presented on MHC II molecules by antigen-presenting cells (APCs), leading to the stimulation of the CD4+ cells to become helper T (TH) lymphocytes. These helper T cells play a critical role in the immune response by stimulating other immune cells or secreting cytokines.