Our body consists of different types of cells which perform specific functions. For example, cells present in kidney help in filtering out the toxic materials from our body, intestinal cells help in process of digestion and alveolar cells in the lung help in process of breathing. All these cells that form various types of organs perform their functions by coordination of various biomolecules like nucleic acid, proteins, lipids and metabolites.
Now, each cell contains certain receptors on its surface for interacting with the outside environment. These receptors can also be used by disease-causing agents like viruses to enter the body. Viruses are very small entities that can only be active when inside the host cells or inside any living system. Viruses usually consist of a unique set of small numbers of genes that help them in making new viral copies. One of the unique things about a virus is it usually targets specific cells in the body. For example, rabies virus targets or infects the neurons and influenza virus targets specifically lung cells.
COVID-19 which is generally known as coronavirus infects those cells which contain a receptor known as Angiotensin Converting Enzyme 2 (ACE2). One of the main functions of the ACE2 receptor is to regulate blood pressure. This receptor is present abundantly in the kidney, lung, intestine, and heart. One of the first organs that COVID-19 infects is the lung. Our lung has two types of cells, cilia, and mucous producing cells. Both these cells help in expelling any foreign unwanted particle from the body. Upon infection by COVID-19 immune cells get activated and kill these virus infected cells and cause the process of inflammation.
One of the outcomes of inflammation is the accumulation of fluid that hampers the gaseous exchange capacity of the lung like impairment in oxygen delivery from the lung to the blood vessel. Inflammatory condition is guided by different types of immune cells that secrete various factors. One of the important factors involved in the inflammatory process is cytokine. Usually, these cytokines are involved in killing or removing the pathogen or disease-causing agent but in the case of COVID-19 infection heightened cytokine response has been observed which is also known as cytokine storm. This further worsens the condition that causes more inflammation and hence more accumulation of fluid which leads to the lack of oxygen in the body.
Lack of oxygen and a plethora of immune responses can initiate a cascade of events that patients feel in the form of symptoms like fever, difficulty in breathing and headache, etc. As we now know COVID-19 enters the cell through the ACE2 receptor and this receptor is present on the surface of other organs also so the chances are through the bloodstream virus can reach these organs. For example, if it will infect the liver cells, it will cause inflammation in the liver which will damage the liver cells and its ability to function.
Blood clotting is another severe problem that has been observed in COVID-19 patients. Therefore, we see that there is a systemic effect of the COVID-19 virus which can lead to multiple organ failure and in worst condition death of the patient. The severity of the disease also depends on various other parameters. For example, heart and kidney problems can make recovery from the COVID-19 infection difficult. Till now, we don’t have a vaccine or specific drug against COVID-19 and timely diagnosis and life-supporting systems like ventilator and dialysis helping patients to recover. The major challenge for the future is to explore the molecular detail of how host cells behave after coronavirus infection and the development of specific drugs or vaccine.