Page 303 - Color Atlas of Biochemistry

294 Tissues and organs
Immune response
Viruses, bacteria, fungi, and parasites that en-
ter the body of vertebrates of are recognized
and attacked by the immune system.Endog-
enous cells that have undergone alterations—
e. g., tumor cells—are also usually recognized
as foreign and destroyed. The immune system
is supported by physiological changes in
infected tissue, known as inflammation.This
reaction makes it easier for the immune cells
to reach the site of infection.
Two different systems are involved in the
immune response. The innate immune system
is based on receptors that can distinguish
between bacterial and viral surface structures
or foreign proteins (known as antigens)and
those that are endogenous. With the help of
these receptors, phagocytes bind to the patho-
gens, absorb them by endocytosis, and break
them down. The complement system (see
p. 298) is also part of the innate system.
The acquired (adaptive) immune system is
based on the ability of the lymphocytes to
form highly specific antigen receptors “on
suspicion,” without ever having met the cor-
responding antigen. In humans, there are sev-
eral billion different lymphocytes, each of
whichcarries adifferent antigenreceptor. If
this type of receptor recognizes “its” cognate
antigen, the lymphocyte carrying it is acti-
vated and then plays its special role in the
immune response.
In addition, a distinction is made between
cellular and humoral immune responses. The
Tlymphocytes (T cells) are responsible for cel- tein that perforates the target cell’s plasma
lular immunity. They are named after the thy-
mus, in which the decisive steps in their dif-
ferentiation take place. Depending on their
function, another distinction is made be-
tween cytotoxic T cells (green) and helper T
cells (blue). Humoral immunity is based on
the activity of the B lymphocytes (B cells, light carry antigen receptors matching those of the
brown), which mature in the bone marrow.
After activation by T cells, B cells are able to (11) and finally secrete large amounts of
release soluble forms of their specific antigen soluble antibodies (12).
receptors, known as antibodies (see p. 300),
into the blood plasma. The immune system’s
“memory” is represented by memory cells.
These are particularly long–lived cells that
can arise from any of the lymphocyte types
described.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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A. Simplified diagram of the immune
response
Pathogens that have entered the body—e. g.,
viruses (top)—are taken up by antigen-pre-
senting cells (APCs) and proteolytically de-
graded (1). The viral fragments produced in
this way are then presented on the surfaces of
these cells with the help of special membrane
proteins (MHC proteins; see p. 296) (2). The
APCs include B lymphocytes, macrophages,
and dendritic cells such as the skin’s Langer-
hans cells.
The complexes of MHC proteins and viral
fragments displayed on the APCs are recog-
nized by T cells that carry a receptor that
matches the antigen (“T-cell receptors”; see
p. 296) (3). Binding leads to activation of the T
cell concerned and selective replication of it
(4, “clonal selection”). The proliferation of im-
mune cells is stimulated by interleukins (IL).
These are a group of more than 20 signaling
substances belonging to the cytokine family
(see p. 392), with the help of which immune
cells communicate with each other. For exam-
ple, activated macrophages release IL-1 (5),
while T cells stimulate their own replication
and that of other immune cells by releasing
IL-2 (6).
Depending on their type, activated T cells
have different functions. Cytotoxic T cells
(green) are able to recognize and bind virus-
infected body cells or tumor cells (7). They
then drive the infected cells into apoptosis
(see p. 396) or kill them with perforin, apro-
membrane (8).
B lymphocytes, which as APCs present viral
fragments on their surfaces, are recognized by
helper T cells (blue) or their T cell receptors
(9). Stimulated by interleukins, selective clo-
nal replication then takes place of B cells that
pathogen (10). Thesematureinto plasma cells