Immunoglobulins (Ig) or antibodies are proteins secreted by white blood cells (B lymphocytes), which circulate in the body and tag, destroy, and/or neutralize bacteria, viruses, and other harmful or foreign materials, the antigens.

Secretory IgA (sIgA) is secreted at the mucosal surfaces such as the mouth, nose, gastrointestinal tract, and can be measured in saliva. SIgA is the first line of defense against infection at these surfaces, acting to prevent colonization by microbes. It is considered particularly key in the defense against viral and bacterial infections of the upper respiratory tract (URTIs), such as colds and influenza.

Scientists at the University of Birmingham (Edgbaston, UK) and their colleagues collected saliva and data derived from the oldest of three age cohorts in the West of Scotland. Of the 1,042 participants who took part in the first wave of the study, 723 were re-interviewed at the third wave, the baseline for the analysis; 91 participants died before wave three leaving 951 participants eligible for participation. Participants were interviewed in their own homes by nurses who were trained in a standardized manner in how to administer a standardized interview schedule and collect the saliva sample.

Saliva samples were taken at the end of the interview but before taking any other physical measures using a standard salivette (Sarstedt Ltd., Leicester, UK). All samples were frozen within two hours of collection and remained frozen at -20 °C until assay. Samples were recovered after thawing by centrifugation at 1,000 g for 10 minutes. Secretory IgA concentration was measured by double antibody sandwich enzyme-linked immunosorbent assay (ELISA). The 639 participants' saliva was sampled in 1995 when they were aged 63 years. Their IgA secretion rate was measured and the mortality rate was tracked over the following 19 years.

The team found that there was a negative association between log sIgA secretion rate and all-cause mortality. Further analysis of specific causes of mortality revealed that the all-cause association was due to an underlying association with cancer mortality and in particular with cancers other than lung cancer. The hazard-ratio (HR) for non-lung cancer was 0.68 implying a 32% reduction in mortality risk per standard deviation rise in log sIgA secretion rate. Higher secretion rates of sIgA were associated with a decreased risk of death from cancer, specifically non-lung cancer, as well as from respiratory disease. Thus, it appears that sIgA plays a protective role among older adults, and could serve as a marker of mortality risk, specifically cancer mortality.

Anna C. Phillips, PhD, the lead author of the study, said, “There are a number of factors that can affect how well we produce antibodies and maintain their levels. It is not yet known how saliva samples could be used in checkups, as scientists have yet to establish what secretion rate would be considered the threshold level before becoming a cause of concern, otherwise known as the "protective level. If the level is very low, this could be considered a useful early indicator of risk.” The study was published on December 23, 2015, in the journal Public Library of Science ONE.

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