Lymphocytes extravasate into tissue and divide to initiate an immune response when encountering a viral antigen.

“well lymphocytes are patrolling the body they encounter this viral antigen in the respiratory epithelium they stop at that location and you can visualize this in mouth ears with something called two Photon microscopy they stop at that location and you can see them extravasate into the tissue when they're there they then begin to divide as they divide the dividing cells can further recognize the viral protein and starts making molecules that can destroy the infected cells but also call other cells into that Arena macroasia neutrophils and so on”