The Earth’s ecosystem is a built around the need for adaptation in order to use energy from the sun. While physical features, mountains, oceans or the seasons, have settled into a rhythm, the Earth’s vegetation has evolved to capture and store the sun’s energy. The diversity of the animal kingdom has then organised itself to exploit, directly or indirectly, these plant and vegetable resources.

Animals, however, also bypass the plant intermediary and sometimes use the sun’s bounty by themselves. The simplest instance is in the use of light for vision. A more specialised one is the absorption of sunlight energy through the skin of animals, which becomes the power source for synthesis, primarily of vitamin D.

Yet another instance has now emerged, of supporting the immune system, and this has been detailed in the Nature group journal, Scientific Reports, by Thieu X Phan, Barbara Jaruga, Sandeep C Pingle, Bidhan C Bandyopadhyay and Gerard P Ahern, of Georgetown University and the Veteran Affairs Medical Centre, both in Washington, and the department of biology, Vinh University, Vietnam.

The role of sunlight in the synthesis of vitamin D is well known and it is the ultra violet part of the spectrum, in sunlight, that is effective. Therefore, we need direct sunlight at preferably midday, and on the skin. Rays from the slanting sun during winter months in the northern countries, for instance, are not useful, which is why people in those countries rush to the beaches in summer.

The manufacture of vitamin D takes place in the inner parts of the thin, outer layer of the skin called the epidermis. Near ultra violet — or light only a little past the blue side of the visible spectrum — is able to penetrate to this part of the skin. These photons are a little more energetic than those of visible light and they convert 7-dehydrocholesterol, a form of cholesterol and a major component of body cells, into a precursor of vitamin D, which changes to vitamin D within about a day. There is ample supply of cholesterol and a short exposure to bright sunlight is all it takes to for a person to generate a day’s requirement of vitamin D. It thereafter plays an important role in the body, by enabling the absorption of calcium and strengthening the bones and similarly several other processes including the immune system.

While plants generally do not produce vitamin D, the earliest source was phytoplankton in the sea over 500 million years ago. That was the source of vitamin D for early, ocean bound vertebrates, but land-based animals needed their own source to sustain their calcium-rich skeletal structure. Vitamin D synthesis using sunlight has therefore been around for 350 million years.

What the Washington and Vietnam based group has discovered is yet another role of the sunlight that gets past the outer layer of skin.

The layer just below the epidermis, called the dermis, is rich in T cells, which are an important part of the immune response of the body. T cells are abundant in the skin, which contains, in fact, twice the number of those in the blood stream. This may be just as it should be, as the skin is the body’s largest organ and the gateway used by myriads of microbes. T cells are either “helper” T cells, which enable the action of other parts of the immune system, or they are “killer” T cells, which destroy virus infected or tumorous cells. And then there are memory T cells, which persist with a record of specific antigens even after a first attack has been overcome.

What the researchers have discovered is that sunlight, which gets through to the T cells, has the effect of increasing their motility or movement.

As T cells, which act against infection, need to move to the site of infection, an increase in their capacity to move is an increase in their capacity to maintain body immunity. While the epidermis is opaque to ultra violet light, blue light, which is the part of the spectrum just below ultra violet, is able to get through and reach the part that is rich in T cells. The Scientific Reports paper notes that blue light passes through the dermis and is reflected back by the blood vessels. So the inner layer of the skin is doubly bathed in blue light.

That apart, the study notes that the T cells in the dermis affected by blue light amount to the discovery of a new type of photo-sensitive cell, apart from the rods and cones of the eye. The researchers found that T cells contain the same components that are found in the cells of the retina, which are light sensitive. Laboratory trials with T cells then showed that the cells were sensitive to UV and blue light, responding with generation of hydrogen peroxide, which leads to higher levels of calcium ions — they are important carriers of signals within and between cells. It is the change of calcium ion concentration that brings about contraction of muscles, and in T cells, irradiation with blue light led to changes in shape and faster random movements of the cells.

Hydrogen peroxide is also the substance that white blood corpuscles release to trigger immune response when they sense infection. “It all fits together” Ahern, from Georgetown University is reported to have said.

Exposure to sunlight is already celebrated for production of vitamin D, and to be more effective than the use of dietary supplements. What we now have is a mechanism by which sunlight promotes the immune reaction of the body, and one more reason for each one of us to go out in the sun. There is a great deal of needless suffering and health care expense that could be avoided if people systematically made sure that they get a short, daily dose of sunshine.