Why Do Some People Infected with COVID-19 Not Produce Detectable Antibodies?
Why Do Some People Infected with COVID-19 Not Produce Detectable Antibodies?
According to the principles of immunology, the immune system consists of two major components: innate immunity and adaptive (acquired) immunity. When SARS-CoV-2 enters the body, it first encounters the innate immune system, which serves as the body’s initial line of defense against infectious diseases. The more effectively this system responds, the greater the likelihood that the virus will be eliminated before it has a chance to spread extensively.
Cells of the innate immune system produce interferons and other inflammatory mediators that can significantly inhibit the early replication of the virus. However, an excessive or uncontrolled innate immune response may also contribute to lung tissue damage. Therefore, maintaining a balanced innate immune response is essential for effective protection. Macrophages and dendritic cells play key roles during the early stages of infection by producing interferon (IFN) gamma and other pro-inflammatory cytokines, including various interleukins, which help combat viral replication. If the antiviral activity of the innate immune system is sufficiently strong, it may suppress the virus during the initial phase of infection without requiring a substantial adaptive immune response. As a result, although a person may have been exposed to the virus, their adaptive immune system may not become significantly activated, and detectable antibodies may never be produced.
Besides the Immune System, What Other Factors Influence COVID-19 Infection?
In addition to the strength of the immune response, two other important factors influence susceptibility to COVID-19: the number of viral receptors expressed on the body’s cells and the amount of virus (viral load) to which an individual is exposed.
Research published in Frontiers in Pediatrics reported that SARS-CoV-2 requires interaction with two proteins located on the surface of human cells to gain entry: ACE2 (Angiotensin-Converting Enzyme 2) and TMPRSS2 (Transmembrane Serine Protease 2). The coordinated interaction of the virus with these two proteins activates the viral spike protein, enabling the virus to enter host cells. Studies have shown that certain lung cells, specialized nasal epithelial cells, and intestinal absorptive cells express relatively high levels of both ACE2 and TMPRSS2, making these tissues more susceptible to viral infection.
Lower expression of these receptors may allow the innate immune system to control the infection more effectively before extensive viral replication occurs. Evidence also suggests that the expression of ACE2 tends to increase with age, particularly after the age of 40, which may partially explain why older adults are generally at greater risk of developing severe COVID-19. When a large number of viral particles enter cells through ACE2 receptors, the innate immune response can become overwhelmed, leading to increased production of inflammatory cytokines and activation of the adaptive immune system, including antibody production.
