Final answer:
Antigen-presenting cells (APCs) efficiently internalize antigens, process them, and present peptide fragments bound to MHC II molecules to trigger an immune response by helper T cells.
Step-by-step explanation:
Antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells, are highly specialized in internalizing antigens, processing them into peptide fragments, and displaying these peptides bound to an MHC II molecule on their membrane. This process begins with the internalization of extracellular antigens by endocytosis. Subsequently, the antigens are degraded within a fusion of a lysosome and the internal vesicle, known as a phagolysosome. The peptide fragments are then loaded onto MHC II molecules. These complexes are transported to the cell surface, where they can be recognized by helper T cells, which are crucial for the adaptive immune response.
Professional antigen-presenting cells are critical for initiating a T cell immune response against pathogens exhibiting extracellular life stages, such as many bacteria, parasites, and fungi. Class II MHC molecules present processed foreign antigens to helper T-cells, signaling the presence of a 'non-self' invader, which leads to the activation and proliferation of the immune response to clear the infection.