So respiratory viruses like the cold, flu, and covid enter the body through the nasal passages and not through the blood stream. Since the white blood cells are in the blood stream, how do they get to the virus to fight it?
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Your respiratory infection symptoms are your immune system fighting the virus at those sites! So you have layers of protection in your nose/throat/lungs similar to your skin (mucosa), but with coats of mucous/secretions to help keep things wet. You also have white blood cells (WBCs) on the surface of this mucosa that are always on the hunt for infected cells.
When a virus lands on a cell in your nose, it goes inside of that cell, makes a bunch of copies, then explodes out to infect nearby cells. This goes on until WBCs show up to clean up the mess, and in doing so they pull the fire sprinkler system and cause all of the other healthy cells to make *lots of mucous* to trap the virus and get rid of it (by leaking out of your nose or down the back of your throat where it’s swallowed into the acid-pit that is your stomach).
When a virus lands in your lungs, the exact same thing happens, only this time the fire sprinkler system is thicker and causes you to cough. When you cough, you move this thicker mucous up through your lungs and up into your mouth where it’s either spit out or swallowed. Same goal though – get the bad stuff out.
[Here’s a neat video](https://www.youtube.com/watch?v=B9Qi7we0Ynk) that shows neutrophils (a kind of WBC) and how they move around the blood stream and wiggle their way through vessel walls into surrounding areas – this is how they can get to surface spots like in the nose/lungs.
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