People who live to 100 years and older have a surprisingly large number of immune cells that destroy cancer cells.

Today, the average human life expectancy is about 71 years.

Only a few people will live to be 100 years old and be called centenarians, and even fewer will live beyond 110 years.

We call such people supercentenarians.

The new study was published in Cell Press journal. This reveals one possible explanation for how these older people may avoid the increasing risk of developing cancer and other diseases as they age.

Around age 100, these lucky individuals appear to begin actively producing a rare form of T-helper cells, expanding and adapting over time.

«"These cells exhibit stepwise differentiation and cytokine plasticity, suggesting adaptive responses to persistent antigens during healthy aging," the study authors write.

We all have T helper cells (also known as CD4 cells), which are part of our adaptive immune system.

There are many different types of CD4 cells, some of which are less common than others. For example, CD4 cytotoxic T cells (CD4 CTLs) are typically quite rare in the human body.

These T cells are more killers than helpers: their job is to patrol the body, identifying infected, damaged, or cancerous cells to destroy them before further damage is done.

However, as people reach age 90 and older, these normally rare CD4 CTL cells appear to become increasingly numerous.

This statistically significant pattern was found in blood samples from a group of 28 Japanese participants (which, admittedly, is a very small sample, consisting of only citizens of one country, and not necessarily representative of the entire world population).

In samples from eight people aged 70, 80, and 90 years, the median CD4 CTL percentage was 4 percent.

Among the 10 study participants who had reached the age of 100, the median rate was 9.6 percent, and among people who had reached the age of 100, the median rate was 17.6 percent.

But even in this small sample, there were outliers: one participant, who was under 100, actually had the highest proportion of these cells in his blood.

These cells were active and showed no signs of exhaustion.

«"Immune aging is not simply a process of decline," says immunologist Kosuke Hashimoto of Osaka University in Japan.

«"The selective expansion of certain T cells suggests that even in old age, the immune system can continue to adapt to age-related changes.".

By studying proteins on the cell surface, the researchers found that these CD4 CTLs arise when helper cells lose the protein CD27 and then CD28.

Killer T cells are known to clone themselves in response to an immune attack, forming a microscopic army.

This is exactly what researchers found in blood samples from older people.

On average, about 33 percent of the CD4 CTLs in their blood were clones of just one original cell.

In a sample taken from one centenarian, about half of his CD4 CTL cells were copies of the same cell.

This suggests that after a certain age, the human body faces increasing threats from the immune system. An excess of CD4 CTLs may help centenarians (at least those living in Japan) survive these threats.

See also: Blood analysis of centenarians revealed 37 proteins associated with slowing down aging.

«"As we age, the number of abnormal cells, including senescent and cancer cells, increases," Hashimoto says.

«"Our results suggest that adaptation of the immune system to these changes may contribute to exceptional longevity.".

The results of the study were published in Cell Press journal .

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