CD40LG and susceptibility to pneumonia measurement: Among them, IgM is the earliest type of antibody produced in the immune response, which activates the complement system and enhances phagocytosis of pathogens, and in children with severe pneumonia, the changes of its level mainly reflect the acute phase characteristics of pathogen infection; while IgA, as a key factor of mucosal local immunity, tends to be distributed on the mucosal surfaces of respiratory and digestive tracts, which can play the role of anti-infection and protective barrier.