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

OVERVIEW OF THE ADAPTIVE IMMUNE SYSTEM
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Dendritic cells of the innate immune system functionally link innate immune
responses to adaptive immune responses. The cells become activated when their
PRRs pick up microbes and their products at sites of infection and phagocytose them.
The activated cells cleave the microbial proteins into peptide fragments, which bind
to newly made MHC proteins, which transport the fragments to the cell surface. The
activated dendritic cells then carry the peptide–MHC complexes to a lymph organ,
where they activate appropriate T cells to make specific adaptive immune responses
against the microbes.
OVERVIEW OF THE ADAPTIVE IMMUNE SYSTEM
A dramatic “big bang” in immune defense mechanisms occurred when jawed vertebrates
evolved and acquired an adaptive immune system. This sophisticated
defense system depends on B and T lymphocytes (B and T cells), which, during
their development, rearrange particular DNA sequences in various combinations
so that, together, the cells can produce an almost limitless variety of B and T cell
receptors and antibodies. Collectively, these proteins can bind to essentially any
molecule, including small chemicals, carbohydrates, lipids, and proteins; individually,
they can distinguish between molecules that are very similar—such as
between two proteins that differ in only a single amino acid, or between two optical
isomers of the same small molecule. By this strategy, the adaptive immune
system can recognize and respond specifically to any pathogen, including new
mutant forms. However, because the genetic rearrangement process produces
both receptors that can bind to self molecules as well as receptors that can bind to
foreign molecules, vertebrates have had to evolve special mechanisms to ensure
that B and T cells do not react against the host’s own molecules and cells—a process
called immunological self-tolerance.
Moreover, many harmless foreign substances enter the body, for example, as
food or inhaled material, and it would be pointless and potentially dangerous to
mount adaptive immune responses against them. Such inappropriate responses
are normally avoided because innate immune responses are required to call
adaptive immune responses into play and do so only when the innate cells’ PRRs
recognize microbial PAMPs, as we discussed earlier. One can trick the adaptive
immune system into responding to a harmless foreign molecule, such as a foreign
protein, by co-injecting a molecule (often of microbial origin) called an adjuvant,
which activates PRRs. This trick is called immunization and is the basis of vaccination.
Any substance capable of stimulating B or T cells to make a specific adaptive
immune response against it is referred to as an antigen (antibody generator).
There are two broad classes of adaptive immune responses—antibody
responses and T‐cell-mediated immune responses, and most pathogens induce
both classes of responses. In antibody responses, B cells are activated to secrete
antibodies, which are proteins that circulate in the bloodstream and permeate
the other body fluids, where they can bind specifically to the foreign antigen that
stimulated their production (see Figure 24–2). Binding of antibody neutralizes
extracellular viruses and microbial toxins (such as tetanus toxin or cholera toxin)
by blocking their ability to bind to receptors on host cells. Antibody binding also
marks invading pathogens for destruction, both by making it easier for phagocytes
of the innate immune system to ingest and destroy them and by activating
the complement system.
In T‐cell-mediated immune responses, T cells recognize foreign antigens that
are bound to MHC proteins on the surface of host cells such as dendritic cells,
which are specialized for presenting antigen to T cells and are therefore referred
to as professional antigen-presenting cells (APCs). Because MHC proteins carry
fragments of pathogen proteins from inside a host cell to the cell surface, T cells
can detect pathogens hiding inside a host cell and either kill the infected cell (see
Figure 24–2) or stimulate phagocytes or B cells to help eliminate the pathogens.
In this section, we discuss the origins and general properties of B and T cells.
In later sections, we consider the specific properties and functions of these cells.