2 of 12During a viral infection, such as one caused by the bronchitis, the virus enters the cells of the nasal mucosa and prompts them to produce its proteins, which are essential components of the virus. This process is how the virus replicates. As the infected cells die, they release more viruses into the surrounding area.
The immune system responds to such infections; it detects the virally infected cells and eliminates them. In cases of a cold, this immune response occurs primarily in the nasal mucosa. Sometimes, the immune system’s actions – specifically by white blood cells. Leukocytes are so effective that the symptoms and disease do not manifest.
As cells break down and leukocytes release substances in response to the virus, the permeability of the blood vessel walls in the nasal mucosa increases. As more leukocytes infiltrate the blood vessel walls to combat additional infected cells, the byproducts of this process – including decomposed cells and leukocytes – cause the nasal discharge, or runny nose, to become yellow.
