CD4 and infection: In lymphoid organs, secondary germinal centres are thought to be formed by reactivated IgM+ memory B‐cells, providing a blank canvas for antibody class switching relevant to the pathogen.73 In influenza virus‐infected mice, CD4 T‐cells and B‐cells can be found in clusters within the lung, and many virus‐specific B‐cells in the lung are class switched.74, 75 This suggests that germinal centres formed in ELS in peripheral tissues may follow different rules to those in lymphoid organs following re‐infection.