Final answer:
The peptide-binding groove of MHC class I molecules is made up of the alpha1 and alpha2 extracellular domains. These domains form the antigen-binding cleft which presents processed antigens to T cells. MHC I and MHC II have different structures and are present on different types of cells.
Step-by-step explanation:
The peptide-binding groove of MHC class I molecules is composed of the following extracellular domains: alpha1:alpha2. Major histocompatibility complex (MHC) molecules are crucial for the immune response, and they exhibit different structures based on their class. MHC I molecules consist of a longer alpha chain that associates with a smaller beta2-microglobulin protein. It is important to note that only the alpha chain of MHC I spans the cytoplasmic membrane and that it folds into three domains: α1, α2, and α3. However, only the α1 and α2 domains are involved in forming the antigen-binding cleft, where processed antigens are presented to T cells.
The antigen-binding cleft of MHC II, on the other hand, is formed by domains α1 and β1. MHC I molecules are expressed on all nucleated body cells, while MHC II are found on specific immune cells such as macrophages, dendritic cells, and B cells. This structural distinction is critical for the specific immune functions each class of MHC molecule mediates in antigen presentation.