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From reading The Pulse, you are probably aware that the immune system changes during pregnancy. You may also be aware that doctors, public health officials, and scientists are concerned about how COVID-19 may interact with pregnancy, specifically whether pregnant women and their fetuses are at particular risk. Although available COVID-19 data seem to suggest that pregnancy itself is not a particular risk for severe complications of the disease, past experience with other coronaviruses (specifically the viruses that cause SARS and MERS), suggest that there could possibly be some pregnancy-related dangers that future study may reveal. On top of that, the rationale for being concerned in the first place is based on knowledge that immune function changes throughout pregnancy, and changes such that defenses against respiratory infections could be lower at certain points of pregnancy as compared with the non-pregnant state. A few years ago, scientists discovered that immune changes actually follow a particular schedule during pregnancy. This may have to do partially with the fact that either the immune system must learn not to attack the developing embryo/fetus as an infection, or the embryo/fetus must be kept isolated from immune system cells.
With this perspective, let’s have a look at the immune system and some of its components and ways that it works. To begin, there is a phenomenon called adaptive immunity, meaning that the immune system is able to adapt to build defenses specific to a particular threat. There is also a phenomenon called innate immunity, meaning that the immune system is also capable of mounting defenses against agents without any kind of learning first. The most obvious element of innate immunity is the skin. As a physical barrier, the skin keeps a lot of potentially disease-causing agents from entering your body. The acid in your stomach also helps with innate immunity as it can destroy a whole range of microorganisms, rather than adapting to fight off any particular one. But innate immunity also involves a range of chemicals and immune cells, providing several opportunities for treatments that can modulate the immune response. Key to the chemicals of innate immunity is the compliment system, which consists of several small proteins, which are altered chemically to create other chemicals called cytokines. Working in concert with all of these chemicals, innate immunity also includes a host of while blood cells, such as monocytes, macrophages, basophils, mast cells, eosinophils, dendritic cells, Langerhans cells, neutrophils, and natural killer (NK) cells. The innate immune system works in concert with the adaptive immune system but also defends the body on its own.
The key to adaptive immunity are a family of white blood cells called lymphocytes. Unlike the various chemicals and cells mentioned above that can attack anything, lymphocytes can recognize things that are foreign to the body and the system can adapt, learning to make more lymphocytes that recognize, and help neutralize, those foreign things better and better. In learning to recognize particular foreign entities (called antigens in immunology), one subtype of lymphocytes called T-lymphocytes mediate and control the adaptive immune response. The other subtype of lymphocytes are B-lymphpcytes. These cells have proteins on their surfaces called receptors, which recognize antigens (foreign entities) by fitting onto them like a key fitting into a lock. When a B lymphocyte does this, the interaction causes the cell to divide, producing more specialized daughter cells called plasma cells, which make antibodies against the same antigen that the ancestor B cell recognized.
After being exposed to a foreign entity (antigen), the adaptive immune system is able to trigger the production of particular B lymphocytes, leading to plasma cells and antibodies that match the antigen. There also are different families of antibodies whose production depends on timing and location within the body. In secreted substances, such as saliva and breast milk, lymphocytes make a type of antibody called IgA. In the blood, the main families of antibodies are called IgM and IgG. When your body encounters an antigen initially, such as when you are infected with virus, or when you receive a vaccine for the first time, most of the antibodies to that antigen are of the IgM family. Over time, however, the immune system makes more of that antivirus antibody in the form of IgG. This phenomenon helps doctors and immunologists tell the difference between a new infection and something that has infected with you a while ago, to which you have developed immunity. As an example, a person with IgM antibodies against SARS-CoV2 (the virus that causes COVID-19) but not many IgG antibodies against that same virus is newly infected. She may be sick and is likely to be infectious to others. On the other hand, someone with IgG antibodies against SARS-CoV2 but not many IgM antibodies was infected weeks to many months ago. Currently, scientists are trying to find out if this means that such a person could not develop COVID-19 a second time and could not spread the disease to others.