COVID-19 Clinical Progression related to Lines of Immune Defense: First Line of Defense to the Last Stand

SARS-CoV-2 virus enters the nose or mouth by airborne or hand to face transmission. It finds cells with ACE2 receptors and settles in to replicate. In children, it may multiply with limited effect because of tissue interferon and nonspecific antiviral immunity, suppression of severe non-specific immunity, and finally resolve with local specific (adaptive) immunity. In adults with mild or no symptoms, it may follow a similar course. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7227586/pdf/main.pdf and https://science.sciencemag.org/content/sci/369/6504/718.full.pdf. If the numbers of virions continues to increase unabated, some will descend into the lungs and/or the gastrointestinal tract. In the lungs and GI tract, the virus infects most of the lumen lining cells and some blood vessel lining cells (endothelium), the latter releasing virus into the blood to spread to other organs in the body. The lungs are so widely infected that the normally dark lung fields appear like ground glass or cloudy (bacterial pneumonia is lobular, localized and white density) in chest radiographs. The images resemble non infectious hypersensitivity pneumonitis in humans or “walking pneumonia” caused by tiny, cell wall-less bacteria called mycoplasma. This stage could resolve with oxygen and other support from a balanced T cell and B cell (antibody) immune response. A similar course with nausea and diarrhea occurs in the GI tract but with more likely limited resolution and generating a fecal shedding carrier state. If the virus continues to replicate and spread, the widely spread virus is confronted by the nonspecific, Type-2 anti-parasite-like and/or Type-3 anti- bacterial and fungal immunity, which will kill the virus but cause varying degrees of collateral damage from asthma-like effects to systemic septic-like shock and perhaps death. This last phase will require ventilators, ICU care and other heroics to survive. The last two steps may result in long term or permanent lung, kidney and other internal organ damage. Wear a mask, distance, sanitize and stay safe!