Final answer:
Antigen peptides fit inside MHC molecule pockets through antigen processing, which cleaves antigens into fragments that can bind within the peptide-binding cleft of MHC molecules, facilitating T cell recognition.
Step-by-step explanation:
The ability for antigen peptides to fit inside the pocket of MHC molecules is facilitated by the processes of antigen processing and antigen presentation. Antigen processing involves enzymatically cleaving antigens into smaller pieces that are then bound to MHC molecules inside the cell. This complex of MHC molecule and antigen fragment is then translocated to the cell surface. The peptide-binding cleft of MHC molecules is structurally adaptable to accommodate a variety of antigens based on their amino acid sequences. The presence of MHC class I and MHC class II molecules, which present intracellular and extracellular antigens respectively, is crucial for T cells to recognize and mount an immune response.
Professional antigen-presenting cells, such as dendritic cells and macrophages, internalize extracellular antigens via receptor-mediated endocytosis and display them with MHC class II molecules. Similarly, all nucleated cells present self-antigens or pathogen-derived antigens via MHC class I molecules. The process by which peptides bind within the antigen-binding cleft of MHC molecules is essential for the discrimination between self and non-self, enabling the immune system to precisely target infected or abnormal cells.