Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1306 Chapter 24: The Innate and Adaptive Immune Systems
ACTIVATED T CELLS MIGRATE TO
SITE OF INFECTION VIA THE BLOOD
skin
MHC
protein
remnants of microbe
in phagolysosome
activated
dendritic
cell
activated T cell
microbes
dendritic cell
microbial peptide bound
to MHC protein
co-stimulatory
protein
lymph node
MICROBES ENTER THROUGH
BREAK IN SKIN AND ARE
PHAGOCYTOSED BY
DENDRITIC CELL
ACTIVATED DENDRITIC CELL
CARRIES MICROBIAL PEPTIDES
TO LOCAL LYMPH NODE
ACTIVATED DENDRITIC CELL ACTIVATES
SPECIFIC T CELLS TO RESPOND TO MICROBIAL
PEPTIDES BOUND TO MHC PROTEINS ON
DENDRITIC CELL SURFACE
INNATE IMMUNE RESPONSE
ADAPTIVE IMMUNE RESPONSE
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 MBoC6 peptides m25.05/24.12 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).
recruit specific and more powerful adaptive immune responses to help fight the
infection. Innate immune responses rely on the ability of host cells to recognize
characteristic features of microbial molecules called pathogen-associated molecular
patterns, or PAMPs, which can be associated with a pathogen’s proteins, lipids,
sugars, or nucleic acids. PAMPs are mainly recognized by pattern recognition
receptors (PRRs), including the Toll-like receptors (TLRs) found on or in both plant
and animal cells. In vertebrates, some PRRs are secreted and can activate complement
when they bind microbial PAMPs. The complement system, which can also
be activated by antimicrobial antibodies bound to pathogens, consists of a group
of blood proteins that are activated in sequence to help fight infections, by disrupting
the pathogen's membrane, stimulating an inflammatory response, or targeting
the microbe for phagocytosis—mainly by macrophages and neutrophils. The
phagocytes use a combination of degradative enzymes, antimicrobial peptides, and
oxygen-derived toxic molecules to kill invading pathogens; in addition, they secrete
various signal molecules that help trigger an inflammatory response.
Cells infected by a virus produce and secrete type I interferons (IFNα and IFNβ),
which induce a complex set of host-cell responses that inhibit viral replication. The
interferons also enhance the killing activity of natural killer (NK) cells. An NK cell
kills infected host cells because they express large amounts of surface proteins that
activate the NK cell; the killing is especially efficient when infected cells express
reduced amounts of class I MHC proteins, which, when present in normal amounts
on a host cell surface inhibit the killing activity of NK cells.