December 29, 2021, 4:11 am | Read time: 5 minutes

Toddlers are constantly catching colds and seem to bring home every infection in the vicinity. Nevertheless, children’s immune systems are by no means weaker than those of adults, researchers report in the journal Science Immunology after conducting experiments with young mice and human cells.¹ In particular, the T cells of the immune system reacted quickly and effectively when they encountered a previously unknown virus. This might explain—among other factors—the often milder course of COVID-19 infections in children.

Everything Babies Encounter Is New

The children’s immune system has a reputation for being weak and underdeveloped, says study leader Donna Farber from the Columbia University Medical Irving Center in New York. But that’s not true, even though babies and toddlers often suffer from viral respiratory infections, such as those caused by the RSV virus (Respiratory Syncytial Virus). This is simply because babies are encountering these viruses for the first time. “Adults don’t get sick as often because we have memories of these viruses stored that protect us,” Farber says. “For babies, however, everything they encounter is new.”

To examine the capabilities of the children’s immune system more closely, researchers collected T cells from young and adult mice. These included cells that had not yet encountered a pathogen, known as naive T cells. T cells are specialized defense cells. There are different types of them; for example, some can recognize and kill virus-infected cells, while others store the memory of individual pathogens to initiate defense measures more quickly upon reinfection. The researchers then administered these T cells to mice, which they subsequently infected with an influenza virus, the cause of the flu.

Also interesting: Infections that particularly affect children

Naive T Cells Multiply Faster

The naive T cells from young mice responded to much smaller amounts of the virus than those from adults. They multiplied faster and migrated in greater numbers to the lungs, where influenza viruses primarily replicate. The researchers were surprised that the cells reacted differently depending on age. “This means that the toddler’s immune system is robust and efficient and can eliminate pathogens at an early age,” Farber says. “In some ways, it may even be better than an adult’s immune system because it is designed to respond to a variety of new pathogens.”

“Early childhood is the time when particularly many T cells are formed in the thymus,” explains Marcus Peters, head of the Immunology of the Lung research group at Ruhr University Bochum.² “As we age, fewer and fewer T cells are formed, and thus the ability to respond to new things decreases.” Adults also have a reservoir of naive T cells, but a smaller one. However, older people have a standard reservoir of memory T cells that has grown over their lifetime. These are tailored to the immune response to specific pathogens. Upon renewed contact with already known pathogens, their immune system can react quickly.

Also interesting: “The immune system works best when left alone!”

How Does the Children’s Immune System Fare with COVID-19?

The children’s ability to quickly respond to new threats may pay off in the case of SARS-CoV-2. The virus is new to everyone, so we are currently witnessing a direct comparison between the children’s and adults’ immune systems. “And the children are doing better.” Older adults confronted with a new virus react more slowly. This gives the virus more time to replicate, and one becomes ill.

That children, despite their very active and powerful immune systems, often suffer from mostly harmless infections more frequently than adults has a simple reason. It is simply because the children’s immune system also needs some time to build an immune response, explains immunologist Peters. “This then goes hand in hand with symptoms of illness.” However, fever or swollen nasal mucous membranes do not indicate a failure of the immune system; on the contrary, they show that it is working well.

Also interesting: How to tell if you have a weak immune system?

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Can Vaccinations Overwhelm Children’s Immune Systems?

Their work also provides clues as to why vaccinations in childhood are particularly effective, the researchers led by study leader Donna Farber continue. One should not worry about receiving multiple vaccinations during this time. “Every child living in the world, especially before we started wearing masks, is exposed to a large number of new antigens every day.” The children’s immune system is accustomed to dealing with this.

“The antigens in the vaccines that trigger an immune response only activate the T cells that are exactly matched to them. T cells that react to the coronavirus, for example, do not react to pneumococci.” Therefore, there is no overloading of the immune system through vaccinations, says immunologist Peters. In general, the immune system is constantly active and continuously deals with antigens that enter the body from the environment, both harmless and pathogenic.

Source

• 1. Puspa, T., Guyer, R.S., Yang, A.Y. et al. (2021). Infant T cells are developmentally adapted for robust lung immune responses through enhanced T cell receptor signaling. Science Immunology.
• 2. Ruhr University Bochum. AG Immunology of the Lung. (accessed on 12/28/2021)

With material from dpa