Final answer:
The ligand that binds to receptor NKG2D on IELs and causes their activation is usually a stress-induced molecule like MICA/B or ULBPs. When these ligands bind, they induce conformational changes leading to intracellular signaling and immune response.
Step-by-step explanation:
The ligand that binds to receptor NKG2D on the surface of IELs (intraepithelial lymphocytes), leading to IEL activation, is generally a stress-induced ligand. These ligands can be up-regulated on infected, transformed, or stressed cells and include MICA/B (MHC class I-related chain A and B) and UL16-binding proteins (ULBPs) amongst others. NKG2D is a C-type lectin-like activation receptor expressed on the surface of NK cells, certain T cells, and IELs which, upon binding with these ligands, can trigger cytotoxicity and cytokine production, thus playing a critical role in immune surveillance.
During the process of receptor signaling, ligand binding prompts conformational changes in the extracellular domain that translate to structural alterations in the intracellular domain. This modification enables the activation of intracellular signaling pathways, which can lead to various responses, such as proliferation or apoptosis, depending on the particular receptor-ligand interaction. Such intricate interactions are essential for the regulation of immune system responses against potential threats.