The adaptive immune system functions mutually with the innate immune system and mediates immune responses selectively targeted to individual pathogens. The response time is delayed, occurring hours to days after exposure to pathogens or foreign molecules. Moreover, the adaptive immune system has the capacity for memory, recording experiences with pathogens and foreign molecules. The memory of immune cells leads to effective and rapid immune responses upon subsequent exposure to the same or similar pathogen. The adaptive immune system comprises specialised immune cells (T cells, B cells, and antigen presenting cells) and antibodies (Marshall et al., 2018). Infants show impaired adaptive immunity compared with older children and adults because immune cells of the adaptive immune system are naive to most pathogens at first. The cells need to experience exposure before they will recognize similar pathogens later in life (Simon et al., 2015).