Sexual HealthWhat Is The Function Of Lymph Nodes?
If we imagine our immune system to be a police force for our bodies, then previous work has suggested that the Lymph nodes would be the best candidate structures within the body to act as police stations - the regions in which the immune response is organised. However, a new paper - published in this week"s issue of PLoS Biology - suggests that lymph nodes are not essential in the mouse in marshalling T-cells (a main immune foot soldier) to respond to a breach of the skin barrier. This result is both surprising in itself, and suggests a novel function for the liver as an alternate site for T-cell activation.
When a child falls off its bike and scratches its skin, the body responds
via the immune system. Scavenger cells at the site of the wound pick up
antigens -tiny particles derived from invading microorganisms and dirt
that the body will recognize as foreign. These antigens are delivered to
the
nearest lymph node. T and B cells (immune cells) carrying the matching
antigen-receptors on their surface will be stimulated by the concentrated
antigen now present in these lymph nodes. T cells will then go on and
orchestrate the defensive response against the invaders, whereas B cells
will
transform into antibody-producing cells flooding the body with antibodies
which act against the hostile microorganisms.
Mice that lack lymph nodes due to a genetic mutation (alymphoplasia) are
severely immuno-compromised and struggle in fighting infections and
tumors.
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New work by Melanie Greter, Janin Hofmann and Burkhard Becher from the
Institute of experimental Immunology at the University of Zurich reports
that
the immunodeficiency associated with alymphoplasia is not due to the lack
of lymph nodes, but caused by the genetic lesion on immune cells
themselves.
The new paper shows that in the mouse T cell function is unperturbed in
the absence of lymph nodes, whereas B cell activation and antibody
secretion
is strongly affected. That T cell responses can be launched outside of
lymph nodes is highly surprising, because this means that T cells can
encounter
antigens elsewhere in order to become activated. By tracing the migration
of fluorescent particles from the site of antigen invasion (i.e. the
wound)
the scientists discovered that the liver could serve as a surrogate
structure for T cell activation. During embryonic development, the liver
is the
first organ to provide us with blood and immune cells. Apparently, at
least in the mouse the liver continues to serve as an "immune organ" even
during adulthood.
This work suggests an explanation for the curious fact that patients
receiving a liver transplant sometimes inherit the donor"s allergies and
immune
repertoire, so in keeping with the idea that donor immune information is
being transplanted. It also suggests that the liver as an immune organ is
an
evolutionary remnant from the time before lymph nodes developed in higher
birds and mammals. Cold-blooded vertebrates have functioning T and B cells
but no lymph nodes. The main achievement of the development of lymph nodes
in mammals is a drastic improvement for the production of better
antibodies. T cells on the other hand have not changed their function much
during evolution and the work by the Zurich group finally provides solid
evidence for the versatility and promiscuity of this cell type.
Funding: This work was supported by grants from the Swiss National Science
Foundation , the National centre for excellence in
Research, the Swiss MS Society, and a
nonrestricted grant from Merck-Serono Pharma Geneva. The funders had no
role in
study design, data collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests statement: The authors declare that no competing
interests exist.
Citation:
"Neo-Lymphoid Aggregates in the Adult Liver Can Initiate Potent Cell-Mediated Immunity."
Greter M, Hofmann J, Becher B (2009)
PLoS Biol 7(5):e1000109. doi:10.1371/journal.pbio.1000109
Plos Biology