Specifically, following the first and second QIV challenge, apart from the serum total and the neutralizing-virus influenza-specific IgG titre (which are commonly used as the end-point immune correlates of protection), draining lymph nodes (dines) and spleens were investigated for the frequency of GC B lymphocytes and GC CD4+ (T helper, Th) lymphocytes. Here, CD4 is linked to influenza.