In Damn science, We have already told you how the body makes antibodies and to what extent they would protect us against possible reinfection. But this is not the only immune response generated by our body against SARS-CoV-2. Is also cellular immunity, with which a type of immune system cells (lymphocytes) destroy cells in the body that have been infected by the virus. What differentiates this response from that of antibodies? Would it protect us against possible reinfection? How long does it last? We explain it to you.
How is cellular immunity different from humoral?
José Antonio Navarro-Alonso, a pediatrician and member of the Editorial Committee of the Spanish Association of Vaccination, explains to Damn science to what cellular immunity is one of the two arms of the immune response. The other is humoral immunity.
To understand what both responses consist of, it is convenient to first explain the work of lymphocytes, which are a type of white blood cell that is part of the immune system. There are two main types, as the National Human Genome Research Institute describes: the B’s and the T.
The B makes the antibodies to fight bacteria, viruses and toxins. In Damn science We have already told you about them and how the body makes antibodies. T lymphocytes destroy the cells themselves of the body that have been infected by a virus or that have become cancerous.
Now we go on to explain the two types of responses to our body. While the humoral response is the one that is mediated by antibodies (made by B lymphocytes), cellular immunity is the response of our immune system mediated by T lymphocytes. The vaccines being tested seek to generate both a cellular and a humoral response.
Margarita del Val, virologist and immunologist at the Higher Council for Scientific Research (CSIC), tells Damn science that cellular immunity refers fundamentally to a type of T lymphocytes: antiviral cytotoxic drugs, which “destroy only infected cells and not healthy neighboring cells.”
“This way they prevent millions of new viruses from being produced in them and help to stop the spread of infection in each person already reduce the ability of this person to infect others”, It indicates. And he adds that there are also virus-specific helper T cells that are relevant because they indirectly help all other specialized components of the immune system.
Most COVID-19 patients generate both responses
There are different studies that show that patients infected with SARS-CoV-2 develop a cellular immune response. Most patients generate both humoral and cellular immunity, according to Navarro-Alonso. However, there are some cases in which the body only develops cellular immunity and does not generate antibodies.
Jorge Carrillo, a member of the Spanish Immunology Society and researcher at IrsiCaixa, indicates to Damn science what in few cases only the cellular response is generated and that finding a humoral response without a cellular response is very difficult. It is not very well known why there are people who do not generate antibodies but do generate cellular immunity. According to Carrillo, it is possible that for antibodies to be generated you need more antigen (a substance not recognized by the immune system) than for the cellular response.
How exactly is cellular immunity generated? What happens in the body of an infected person? Del Val explains that there are cells in the areas where infections enter the body, such as the skin and mucosa, which are dendritic cells: “They are professionals in giving early warning. They catch the virus as it enters, dissects it and presents it in fragments to young inexperienced T lymphocytes that are continually circling the body in the blood. ”
So T lymphocytes that can be antiviral “They are activated for the first time and they begin to multiply”. “Some will go to the first line, to destroy infected cells, and others will specialize in memory elite cells that will be much more effective in facing a second infection afterward, already with experience, helping to protect us”, he adds.
One of the big questions is to what extent does the cellular immunity protect against re-infection. For Del Val, protection “is a team effort, of various components of the immune system. “Along with antibodies and other mechanisms of the immune response, cellular immunity to contributes to protection against reinfection. It is still unknown to what degree each immune component does it, “he says.
Measuring cellular immunity requires culturing cells in the laboratory and complex techniques
How are humoral and cellular immunity detected? María Luisa Gil Herrero, professor at the Department of Microbiology and Ecology at the University of Valencia, tells Damn science that humoral immunity can be detected by measuring antibodies in the blood. At present, it is being done by “relatively simple” techniques such as rapid tests.
“Measuring the cellular response is more complicated since it requires culturing cells in the laboratory and the use of more sophisticated techniques that are not developed for a rapid clinical diagnosis ”, says Gil, who is also part of the Spanish Society of Microbiology.
For example, Carrillo indicates that one of the methods to assess cellular immunity It consists of stimulating the cells with specific peptides (amino acids linked by chemical bonds) so that when there are specific cells for the peptide, they activate and express or secrete certain proteins that allow their identification.
How to detect this immunity “is more complex and more expensive”, according to Del Val, it is only being done in a population that is not very representative of patients, not for diagnostic purposes and only for specialized immunologists.
Cellular immunity in blood from patients collected before the pandemic
One study analyzed the blood of healthy volunteers before the pandemic (collected between March 2015 and March 2018). Approximately half had a repertoire of T cells reactive to SARS-CoV-2. The researchers indicate that it could be due to previous exposures to other types of coronavirus and would show a cross-reaction against the COVID-19 virus.
More research is still needed to draw conclusions about it. But, if these results are confirmed, they could help explain the variability of symptoms among people with similar risk factors. Or to understand why children (who may have colds more frequently) are less likely to develop a serious illness, as indicated by the World Health Organization.
In addition, there are many unanswered questions about immunity (both cellular and humoral). For example, in Del Val’s words, it is still unknown how long they last and to what degree they contribute to protection against infection each of these two major components of the immune response. Also how to induce each response optimally with current vaccines, what levels of each response are the minimum to contribute in a significant way to protect ourselves or even if any of these components can contribute to aggravating the disease due to poor function.