Figure 2: Immune development during critical window of vulnerability. (a) Timeline of maturation of bronchus-associated lymphoid tissue (BALT), gut-associated lymphoid tissue (GALT), T cells, and macrophages. (b) Lung-resident SDPCs could be the plausible source of lung DCs in early childhood. The timing of lung DC development, environmental factors triggering this transition, and signaling mechanisms involved in DC development remain unknown.