Final answer:
MHC class II molecules are found on professional antigen-presenting cells and essential in presenting extracellular antigens to T cells. They internalize antigens, combine them with MHC II in vesicles from the Golgi, and then move to the surface to activate T cells.
Step-by-step explanation:
The formation and function of MHC class II molecules involve their expression on the surface of professional antigen-presenting cells (APCs), including macrophages, dendritic cells, and B cells. These molecules are integral for the adaptive immune response, particularly in the activation of T cells.
Extracellular antigens, such as those from bacteria and fungi, are internalized into APCs via receptor-mediated endocytosis. Inside the cell, these antigens are enclosed in vesicles that fuse with other vesicles containing pre-formed MHC II molecules from the Golgi apparatus. The combination of the antigen and MHC II within these vesicles then travels to the cell surface, where the antigen is presented, ready for recognition by T cells.
This process is vital for the initiation of an immune response, as a T cell will only become activated when it encounters a foreign antigen in association with an MHC II molecule on the surface of a professional APC. This leads to the proliferation and differentiation of T cells, essential for effective immunological response against the pathogen.