Dendritic cells are crucially important components of the innate immune system. They are a heterogeneous class of cells that are widely distributed in the tissues and organs of vertebrates. They express a large variety of PRRs, which enable dendritic cells to recognize and phagocytose invading pathogens and their products and to become activated in the process. The activated dendritic cells cleave the proteins of the pathogen into peptide fragments, which bind to newly synthesized MHC proteins, which then carry the fragments to the dendritic cell surface. The activated cells then migrate to a nearby lymphoid organ such as a lymph node (also called a lymph gland), where they present the peptide–MHC complexes to T cells of the adaptive immune system, activating the T cells to join in the battle against the specific pathogen (Figure 24–11).
In addition to the complexes of MHC proteins and microbial peptides displayed on their cell surface, activated dendritic cells also display cell-surface co‑stimulatory proteins that help activate T cells (see Figure 24–11). As we discuss later, the activated dendritic cells also secrete a variety of cytokines that influence the type of response that the T cells make, ensuring that it is appropriate to fight the particular pathogen. In these ways, dendritic cells serve as crucial links between the innate immune system, which provides a rapid first line of defense against invading pathogens, and the adaptive immune system, which mounts slower but more powerful and highly specific responses to attack an invader, as we now discuss.
All multicellular organisms possess innate immune defenses against invading pathogens; these defenses include physical and chemical barriers and various defensive cell responses. In vertebrates, these innate defense responses can also
Figure 24–10 How an NK cell recognizes its target. An NK cell preferentially attacks infected host cells and cancer cells because these cells have on their surface both activating proteins and, in some cases, abnormally low levels of class I Mhc proteins. (A) The high levels of class I Mhc proteins found on normal host cells activate inhibitory receptors on the NK cell that suppress the killing activity of the NK cell. (B) In contrast, the activating proteins on infected cells and cancer cells bind to activating receptors on the NK cell and stimulate the killing activity of the cell.
Figure 24–11 Dendritic cells as functional links between the innate and adaptive immune systems. Dendritic cells pick up invading microbes or their products at the site of an infection. The microbial pAMps activate the dendritic cells to express co-stimulatory proteins and increased amounts of Mhc proteins on their surface and to migrate via lymphatic vessels to a nearby lymph node. In the lymph node, the activated dendritic cells activate T cells that express appropriate receptors for the co-stimulatory proteins and the microbial peptides bound to Mhc proteins on the dendritic cell surface. The activated T cells proliferate, and some of their progeny migrate to the original site of infection, where they help eliminate the microbes, either by activating local macrophages or by killing infected host cells (not shown). In addition, some of the activated T cells help stimulate specific B cells in the lymph node to secrete antibodies against the microbe (not shown).
A crucial feature of dendritic cell activation is that the pathogen provides an individual dendritic cell with both the peptides for presentation to T cells and the pAMp signals that activate the dendritic cell to express co-stimulatory proteins. In this way, the individual dendritic cell has all it needs to activate specific T cells that recognize the peptide–Mhc complexes on its surface (Movie 24.3).