Antigen Presenting Cells (APCs)
Under pathological conditions, for ex-
ample, in skin suffering from the autoim-
mune disease psoriasis, not only LCs but
also keratinocytes express high amounts
of MHC class II molecules and become
capable of functioning as APCs. In other
squamous epithelia, DCs different from
LCs do occur. This is true of the oral and
nasal mucosa, the trachea and bronchus,
and the esophagus and the tonsils. In the
epithelia lining the lung, the stomach,
and the gut, DCs have not been found,
as yet.
APCs in the Mucosa
Although tight junctions and the mucous
layer of mucosal surfaces widely exclude
microbes and large particulate antigen(s),
antigenic proteins are known to surpass
this barrier through endo- and transcyto-
sis. Granulocytes, as representatives of the
innate immune system, and macrophages
functioning as APCs take up these anti-
gens in the lamina propria. Furthermore,
in the gut and bronchus, an antigen trans-
fer system exists that transports antigens
from the lumen to the GALT and BALT re-
spectively. A prerequisite of this system is
the presence of a specialized type of epithe-
lial cell, the so-called
(see below).
APCs in the Gut
The uptake of bacteria, proteins, and
abiotic substances (e.g. latex) by the Peyer’s
patches and by lymphoid follicles in other
parts of the gut, for example, the appendix,
is well known. The GALT is separated
from the gut lumen by a single layer of
epithelium, containing M cells (Fig. 3). In
ultrathin sections, an M cell is seen as a rim
of apical cytoplasm that bridges the space
between two adjacent epithelial cells. An
M cell forms a kind of an umbrella above a
space surrounded by epithelial cells and
±lled with all sorts of APCs including
DCs, macrophages, and B cells. Thus,
antigen from the gut lumen is transcytosed
M cell
B cell
Gut lumen
Dendritic cell
Fig. 3
The gut-associated lymphoid tissue (GALT). Antigens
from the gut lumen traverse the single layer of epithelia via M
cell–mediated transcytosis whereupon they get in contact with
APCs (dendritic cells, macrophages, B cells) of the GALT. APCs
are very similarly organized in the lymphoid tissues of the
bronchus and Waldeyer’s ring.
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