For example, most children infected with Covid-19 will have mild symptoms or no symptoms at all, while adults infected with Covid-19 will become very serious or even fatal.

However, children infected with Covid-19 are at risk for a rare but very serious syndrome, the childhood inflammatory multifactor syndrome (MIS-C). Severe cases of MIS-C can lead to heart disease and ventricular failure, and require hospitalization and increased medical support.

Dr. Galit Alter, scientist, Ragon Institute member of MGH, MIT and Harvard, and Dr. Lael Yonker, director of the Center for Cystic Fibrosis - Massachusetts General Hospital, are currently working together to better understand why Covid-19 is able to lead to such clearly different results in different populations.

In a recent study published in the journal Nature Medicine , the team identified specific types of antibodies that can induce reactions that produce different results with this Covid infection, of which one type antibodies are specific to severe illness in adults and the other is specific to MIS-C in children.

"We found that children who developed MIS-C after becoming ill or exposed to Covid-19 had very high levels of one type of antibody, namely IgG. Normally, IgG is active," Yonker said. is active to control infection, but for MIS-C, IgG strongly activates immune cells, which can lead to serious illnesses, such as MIS-C in children.

"Specifically, the IgG antibodies interact with cells called macrophages - which live in all tissues in the body. When too much IgG antibodies activate these macrophages, it can cause inflammation. In many different organs and systems, this is seen in MIS-C. This high level of IgG antibodies is only found in children who develop MIS-C after becoming ill or exposed to Covid-19. ", Yonker explained further.

Yonker is a pediatric pathologist at MGH and an as‌sistant professor at Harvard Medical School (HMS), running a biological archive that collects samples from cystic fibrosis patients in children. When a pandemic struck, she began to collect samples from children with mildly infected Covid-19.

When Yonker and other pediatricians noticed many children admitted to hospital with the name MIS-C, which would normally onset three to six weeks after developing Covid-19, she also quickly began to collect those samples. She wants to understand how a mild case of Covid-19 can lead to a severe MIS-C episode several weeks after recovery.

To learn more about this immune response, Yonker collaborated with Alter, also a professor at HMS and an immunologist at MGH’s Department of Infectious Diseases. Alter’s team used her unique " systematic serology " technology to make a detailed comparison of the immune response in children - 17 with MIS-C and 25 with Covid- 19 mild forms - with the response of 26 severely ill adults and 34 mildly ill adults.

"We have expected that the immune response in children will be different from those of adults, regardless of the severity of the disease. However, we have found that adults with Covid-19 infection are mild and that children are infected. Covid-19 all have similar immune responses. Only adults with severe Covid-19 have different immune responses, "says Alter.

Alter explains that, for adults with severe Covid-19, the amount of IgA antibodies increases, interacts with a type of immune cell called neutrophils and causes neutrophils to release cytokines. . When there are too many IgA antibodies, the neutrophils can spike, leading to the release of too much cytokine. This could contribute to a cytokine storm, one of the symptoms of Covid-19 severe.

"In MIS-C, high levels of IgG antibodies can trigger macrophages, leading to possible inflammation in the Ragon Institute," said Dr. Yannic Bartsch, the study’s first author and also at the Ragon Institute. In adults with severe Covid-19, high levels of IgA antibodies can prompt neutrophils to release too much cytokines, potentially causing a cytokine storm.

Identifying the immune mechanisms of many different responses to the same virus is the first step in understanding why it induces different responses in different populations.

Discovering how the immune system’s response to disease and its outcomes in both children and adults can help researchers develop treatments that can either suppress or modulate the response. immunity, retains its protective functions but may lessen unintentional but harmful effects.