Final answer:
MHC class I molecules present both self and pathogen-derived antigens to T cells for cellular immunity, while MHC class II molecules, found only on professional antigen-presenting cells, prime T cells by presenting extracellular pathogen antigens.
Step-by-step explanation:
Function of Major Histocompatibility Complex Class I and Class II Molecules
Major histocompatibility complex (MHC) class I and class II molecules have crucial roles in the immune system related to antigen presentation.
MHC class I molecules are found on all nucleated cells and are responsible for presenting normal self-antigens as well as abnormal or pathogen-derived antigens to effector T cells.
This is essential for cellular immunity, especially against intracellular pathogens like viruses that replicate within the host's cells.
These antigens are processed and presented on the surface of the infected cell, triggering a cytotoxic T cell response that can ultimately lead to the destruction of the infected cell.
In contrast, MHC class II molecules are solely located on professional antigen-presenting cells, such as macrophages, dendritic cells, and B cells.
They present foreign antigens originating from extracellular pathogens for the initial activation of T cells, stimulating the adaptive immune response.
Extracellular antigens are internalized into the cell by endocytosis and after processing, they combine with MHC class II molecules before being presented on the cell surface to helper T cells.