Antigen Presenting Cells (APCs)
357
T
H1
Helper T cells that produce high levels of IFN-
γ
and promote activation of macrophages.
T
H2
Helper T cells that produce IL-4, IL-5, and IL-I0 and promote eosinophil, B-cell, and
mast-cell functions.
Tolerance
The failure of the immune system to respond to antigens.
Vaccination
Administering an antigen to generate long-term antibody or cellular immune responses
or both.
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Antigen presenting cells play a key role in the immune system as they are the only
cells of the body that are able to trigger antigen-speci±c immune responses upon
infection with foreign invaders. Three differentially specialized types of cells are
in charge of this vital task: dendritic cells, macrophages, and to a certain extent,
B lymphocytes. We are beginning to understand the molecular mechanisms of
how these cells take up, process, and present antigens and how this translates
into T-cell activation or silencing. This knowledge is fundamental for both our
current concepts about how the immune system discriminates between self and
nonself and for the development of novel vaccination strategies against diseases still
threatening mankind.
1
Antigen-specifc Immunity
Our environment is rich in microbes
and parasites, a multitude of them be-
ing pathogenic to humans and/or to other
mammals or vertebrates. The necessity of
means to combat microbial invaders is evi-
dent and is accomplished through the pres-
ence of a functional immune system. One
type of weapon was discovered by Behring
and Kitasato at the end of the nineteenth
century in the serum of rabbits immunized
with diphtheria bacilli: the cell-free serum
was
capable
of
destroying
diphtheria
toxins, it was speci±c for diphtheria and re-
mained effective in the organism of other
animals – the breakthrough in immuniz-
ing against diphtheria. Behring and his
colleagues named the serum components
of this monumental discovery
antitoxins
.
Later on they were named
antibodies
,and
the agents able to induce the generation of
antibodies became known as
antigens
.
An immune response can be antigen-
speci±c, as in Behring’s immunization ex-
periments with diphtheria and tetanus tox-
ins, but also antigen-nonspeci±c (Fig. 1).
Nonspeci±c immunity is based on special-
ized cells, such as natural killer (NK) cells,
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