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The immune system is a diverse group of cells that recognize and attack foreign substances, pathogenic organisms and
cancer cells. It also produces inflammation, an essential component of the wound healing process and, following the
resolution of inflammation, plays a crucial role in the control of granulation tissue formation. Granulation tissue is the
precursor of scar tissue. Injured skin and mucous membranes generally heal rapidly. However, some wounds are either
slow to heal or fail to heal while in others overgrowth of scar tissue occurs, resulting in the production of either
hypertophic or keloid scars. The modulation of wound healing in such conditions is clinically important and may even
be vital. Evidence will be presented that phototherapy can modulate wound healing, and that changes induced in the
immune system, in particular the secretion of soluble protein mediators including cytokines, may be involved in this
modulation. The immune system has peripheral and deep components. The former, being located mainly in the skin and
mucous membranes, are readily accessible to photons, which can affect them directly. The components of the immune
system are linked by lymphatic vessels and blood vessels, which include many capillaries located in the sub-epithelial
connective tissues of the skin and mucous membranes. The superficial location of these capillaries provides the immune
cells and molecules in transit through them with ready access to photons. When these cells and molecules, some
modified by exposure to photons, reach susceptible cells such as lymphocytes in the deeper parts of the immune system
and cells of injured tissues, they can modify their activity. In addition to having direct effects on peripheral cells,
photons can thus also produce indirect effects on cells too distant for the photons to reach them. For example, cytokines
released from peripheral macrophages in response to the direct action of photons can be transported to and affect other
cells, including fibroblasts of injured tissues, that have not been exposed to photons. It is therefore possible for injuries
other than those directly exposed to phototherapy to be affected by it indirectly.
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