Final answer:
Extracellular antigens are taken up by cells through receptor-mediated endocytosis, processed, and presented on the cell surface in association with MHC molecules. MHC class II molecules are primarily involved in the presentation of these extracellular antigens by antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. This process is essential for the recognition of the antigen by T cells and the activation of the adaptive immune response.
Step-by-step explanation:
Extracellular antigens, such as those characteristic of bacteria, parasites, and fungi that do not replicate within the cytoplasm, are primarily taken up by cells through a process called receptor-mediated endocytosis. Following this, these antigens are processed within the endomembrane system of the cell.
They are enzymatically cleaved into smaller pieces in a process known as antigen processing. The smaller antigen fragments then associate with MHC class II molecules within the Golgi complex. This antigen-MHC complex is then transported to the cell surface in a vesicle, where it can be recognized by T cells, a crucial step in the adaptive immune response.
There are two key MHC classes involved in antigen presentation: MHC class I and MHC class II. MHC class I molecules present antigens derived from intracellular pathogens, like viruses, which are processed in the cytosol and transported to the surface via the endoplasmic reticulum.
In contrast, MHC class II molecules are responsible for presenting extracellular antigens which are processed and then presented on the surface of antigen-presenting cells (APCs).
Professional antigen-presenting cells, such as dendritic cells, macrophages, and B cells, play crucial roles in this process. Dendritic cells, for example, phagocytose pathogens and present the processed antigens with MHC II molecules to T cells for the initiation of the adaptive immune response.