I came across this from the Open University and thought it was a good explanation. Quite detailed, but worth reading - also covers how testing (for both the virus and antibodies) can be done. www.open.edu/openlearn/science-maths-technology/biology/how-does-the-human-body-fight-viral-infection?in_menu=1063877
The font is small reading on a phone but I have just discovered on an iPhone you can adjust font size in the address bar.