Abstract
In this chapter, we will discuss the principles of antigen presentation. We will learn that the major purpose of antigen-presenting cells, such as DCs, macrophages and B cells, is to display peptide antigens to T cells. These peptides are derived from extracellular microbes and are presented via MHC II protein to CD4 + T cells (T H cells), which are found with highest concentration at secondary lymphoid organs, such as lymph nodes. In contrast, antigen peptides that origin from intracellular microbes or damaged cellular proteins are displayed by all kind of body cells on MHC I proteins to CD8 + T cells (cytotoxic T cells). Thus, we will understand that MHC proteins are central in segregating antigens that are produced inside of regular cells, such as viral proteins, from those that internalized from outside, such as proteins from extracellular bacteria.