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

1297
The Innate and Adaptive
Immune Systems
chapter
24
As we discussed in Chapter 23, all living organisms serve as hosts for other species,
usually in relationships that are benign or even mutually helpful. But all
organisms, and all cells in a multicellular organism, need to defend themselves
against infection by harmful invaders, collectively called pathogens, which can
be microbes (bacteria, viruses, or fungi), or larger parasites. Even bacteria defend
themselves against viruses, using intracellular proteins called restriction factors,
which block viral propagation. Invertebrates use a variety of defense strategies,
including protective barriers, toxic molecules, restriction factors, and phagocytic
cells that ingest and destroy invading pathogens. Vertebrates, too, depend on such
innate immune responses, but they can also harness more sophisticated and specific
mechanisms, called adaptive immune responses. The innate responses occur
first, calling the adaptive immune responses into play if required, in which case,
both types of responses work together to eliminate the pathogen (Figure 24–1).
Whereas innate immune responses are general defense reactions that can
involve almost any cell type in an organism, the adaptive immune responses are
highly specific to the particular pathogen that induced them and depend on a class
of white blood cells (leukocytes) called lymphocytes. There are two major classes
of lymphocytes that mount adaptive immune responses—B lymphocytes (B cells),
which secrete antibodies that bind specifically to the pathogen, and T lymphocytes
(T cells), which can either directly kill cells infected with the pathogen or produce
secreted or cell-surface signal proteins that stimulate other host cells to help eliminate
the pathogen (Figure 24–2). Unlike innate immune responses, which are
generally short-lasting, the adaptive responses provide long-lasting protection:
a person who recovers from measles or is vaccinated against it, for example, is
protected for life against measles by the adaptive immune system, although not
against other common viruses, such as those that cause mumps or chickenpox.
Both the innate and adaptive immune systems have evolved sensing mechanisms
that enable them to recognize harmful invaders (pathogens) and distinguish
them from both the host’s own cells and molecules and harmless or beneficial
foreign organisms and their molecules. The innate system relies on sensor
proteins that recognize particular types or patterns of molecules that are common
to pathogens but are absent or sequestered in the host. The adaptive system, by
contrast, uses unique genetic mechanisms to produce a virtually limitless diversity
of related proteins—receptors on T and B cells and secreted antibodies—that,
between them, can bind almost any foreign molecule. This remarkable strategy
enables the adaptive immune system to react specifically against any pathogen,
even if the animal never encountered it before. But, it also requires that the system
learn not to react against self molecules or harmless foreign ones; if these learning
mechanisms fail, harmful autoimmune or allergic responses result.
In this chapter, we focus on vertebrate immune responses and the features
that distinguish them from other kinds of cell responses. We begin with innate
immune defenses and then discuss the highly specialized properties of the adaptive
immune system.
In This Chapter
THE INNATE IMMUNE SYSTEM
OVERVIEW OF THE ADAPTIVE
IMMUNE SYSTEM
B CELLS AND
IMMUNOGLOBULINS
T CELLS AND MHC PROTEINS
PATHOGENS
INNATE
IMMUNE
RESPONSES
ADAPTIVE IMMUNE RESPONSES
Figure 24–1 Innate and adaptive immune
responses. Innate immune responses are
activated directly by pathogens and defend
all multicellular organisms against infection.
In vertebrates, pathogens, together
with the innate immune responses they
activate, MBoC6 also stimulate m25.01/24.01 adaptive immune
responses, which then work together with
innate immune responses to help fight the
infection.