Immune system

The immune system protects the organism from environmental factors such as microorganisms, viruses and toxic substances and serves to overcome diseases. The immune system provides innate immunity and adaptive immunity through a complex network of immune cells, molecules and immunological processes (Marshall et al., 2018). 
Infants have an undeveloped innate immunity at first due to increased permeability of all physical barriers such as lung or intestine (intestinal barrier). The adaptive immune system functionality is at first naive to several pathogens resulting in impaired immune responses until (repeated) exposure and “learning” leads to adequate responses for the right pathogens (Simon et al., 2015).