Final answer:
The mechanism restricting the lateral movement of transmembrane proteins in antigen-presenting cells, leading to the activation of an adaptive immune response, is MHC presentation. Antigens are processed and presented on the cell surface in complex with MHC class I or II molecules to activate T cells.
Step-by-step explanation:
The mechanism by which an antigen-presenting cell triggers an adaptive immune response is best described by MHC presentation. Antigen-presenting cells process the antigens internally and present them on their surface using Major Histocompatibility Complex (MHC) molecules. There are two types of MHC molecules: MHC class I and MHC class II, which present antigens from intracellular sources, like viruses, and extracellular sources, respectively. Antigens processed in the cytosol by proteasomes are presented with MHC class I molecules, and antigens processed by endocytosis, such as phagocytosis, are presented with MHC class II molecules. This presentation is critical for activating T cells, which are essential in the adaptive immune response.
Cells use different pathways for endocytosis: phagocytosis for large particles and receptor-mediated endocytosis for specific molecules, while exocytosis is the process of expelling materials from the cell. However, the direct answer to the student's question is that during an adaptive immune response, the antigen presentation via MHC molecules is the key mechanism restricting the lateral movement of transmembrane proteins.