Final answer:
The molecule that binds to LFA-3 on a cytotoxic T cell is CD8. Cytotoxic T cells engage with MHC class I molecules using CD8 to recognize and destroy infected cells. This function is distinct from that of CD4+ helper T cells which interact with MHC class II on antigen-presenting cells.
Step-by-step explanation:
The molecule that binds to LFA-3 on a cytotoxic T cell is CD8. Cytotoxic T cells, also known as CTLs, use the CD8 coreceptor to engage with MHC class I molecules which are found on all nucleated cells. In contrast to cytotoxic T cells, helper T cells are CD4+ and they interact with MHC class II molecules found on specialized antigen-presenting cells such as macrophages, dendritic cells, and B cells. A cytotoxic T cell recognizes antigens presented by MHC I molecules on infected cells, leading to the activation of the T cell and subsequent destruction of the infected cell using molecules such as granzymes and perforins.
It's important to correct a common misunderstanding: MHC II is not found on most body cells, rather it is present on specialized antigen-presenting cells. Conversely, MHC I is found on all nucleated cells, not just immune cells. The T Cell Receptor (TCR) is an essential part of both CD4+ and CD8+ T cell function, as it is responsible for recognizing antigens presented with either class I or class II MHC molecules.