Final answer:
The difference lies in the cellular processing routes and MHC molecules associated with the antigens. Endogenous antigens are presented by MHC class I and lead to the activation of cytotoxic T cells, while exogenous antigens are processed and presented on MHC class II molecules, resulting in the activation of helper T cells.
Step-by-step explanation:
The difference between the processing of endogenous antigens and that of exogenous antigens lies in the route taken within the cell and the type of major histocompatibility complex (MHC) molecule they associate with for presentation.
Endogenous antigens, which typically come from viruses or other intracellular pathogens, are processed within the cytosol and presented by MHC class I molecules, which are found on all nucleated cells. This process activates cytotoxic T cells, which can lead to the destruction of the infected cell.
Conversely, exogenous antigens, which are generally from pathogens outside cells like bacteria and parasites, are internalized and processed via receptor-mediated endocytosis and placed on MHC class II molecules. These molecules are expressed only by 'professional' antigen-presenting cells — macrophages, dendritic cells, and B cells — and they activate T cells that help orchestrate the immune response.
Thus, the correct answer to the student's question is 'd. endogenous antigens are presented by MHC class I, exogenous by MHC class II'.