I watched many videos and read bunch of answers about this, but I was having a hard time finding an answer to this one point: What is the exact moment an organism realizes that it is under an attack? I understand that there might be a few different types of ways a body can react too. So far, this is my understanding: - Certain amounts of white cells swim around the body, scanning for different movement and chemical "patterns". - They do this by making their own proteins from the DNA that they are being coded from, which look for these "patterns". - When there is anything that they don't recognize, they make the white blood cell attack. - Additionally they are able to call other cells "for help", and they all collectively absorb pathogens and try to dissolve them with their enzyme proteins (with a little help from antibody proteins that bind and neutralize the pathogens). - Plus some of them can remember these patterns by retaining the genetic information of the pathogens, through which they generate the appropriate proteins. I hope I presented the "simplified" picture as good as possible. My question is: What is the exact thing that happens when the antibody protein "recognizes" the antigen entity? Or better, "how" does the antibody "make" the cell (or some other cell) attack? Does it send some kind of a signal back to the cell (or nucleus) in some way? If yes, in what way? Once that "signal" is received by the cell/nucleus, would then the nucleus start coding for new proteins that will do a different action for the cell to get rid of the antigen? Or is this all automatically done by the same protein in the first place? I hope the question makes sense and that I am even thinking of it in the right way. Any thoughts welcome!