Is "exposure" a continuum (e.g. virus units/day)? If it is, then does everyone with anything over a certain intensity of exposure start developing antibodies wi

Is "exposure" a continuum (e.g. virus units/day)? If it is, then does everyone with anything over a certain intensity of exposure start developing antibodies without necessarily being symptomatic? IOW do immune systems develop background protection or does it take becoming symptomatic in order for the immune system to become sensitized?