## 4 Discussion  
The main aim of the present study was to investigate a potential difference in the impact of consumption of the postbiotic FM-CBA74 on the concentration of various immune markers between newborns and young children, more specifically regarding a potential baseline dependent effect. The underlying thought was that the immune system is considered to be in a different phase between both age groups: in the newborns it is starting to develop whereas in young children the system is already in a more matured state. This perspective could affect the relationship between baseline value and the potential of an increase in the various factors. The results of the present study show that the 3-month  
Figure 10: Young children: the association between the change in fecal butyrate and the absolute number of common infectious diseases (CID), both respiratory and gastro-intestinal, at the end of the experimental interval (day 90), as observed in one study ([PERSON], 2017). Each symbol represents one observation, the curve shows the exponential regression curve. Figure 9: Young children: the association between the baseline concentration of fecal butyrate and the change in these concentrations after the 3-months study period. Each value represents one single observation. consumption with CBA L74 exerts a beneficial stimulatory activity on the various immune biomarkers in newborns (\(\alpha\)-defensin and s-IgA) and in young children (all factors) of which three represent the innate (\(\alpha\)-defensin, \(\beta\)-defensin, and cathelicidin) and one (s-IgA) the adaptive component s-IgA. The baseline levels of the immune biomarkers had a significant influence on the increase after the 3-month consumption period: those in the newborns being negative (the higher the value at the start the lower the increase after 3 months) except for \(\beta\)-defensin, whereas those in the young children being positive for only the postbiotic group (the higher the value at the start the larger the increase after 3-month consumption) whereas in the placebo group no impact was observed at all. Acknowledging the fact that the number of participants in the newborn study was substantially lower than that in the studies on young children, clear associations of the increases in the biomarkers were demonstrated for most of the parameters in the latter while in the former only between \(\alpha\)-defensin and \(\beta\)-defensin, and between s-IgA and \(\beta\)-defensin. Overall, consumption of the postbiotic yields higher levels of all immune biomarkers studied in both newborns and young children. Moreover, the effect of the starting levels is found shortly after birth irrespective of the drink and in children 1-4 years of age only after consumption of the postbiotic. To our knowledge, the differentiating influence of the baseline value on the increase in the various immune markers between newborns and young children as observed in the present manuscript is presented for the first time. This phenomenon might be explained by a different maturation stage of the immune system between newborns (initiation stage) and young infants (maturation stage). One of the important characteristics during maturation is the decrease in intestinal permeability [21] leading to a difference in uptake of macromolecules and immunoglobulins between or via the intestinal epithelial cells. Importantly, a direct effect of feeding patterns of breast versus formula milk on tight junction functionality via zonulin has not been established [22]. Another striking feature is the enhanced endocytosis capacity of immature intestinal epithelial cells shortly after birth [23], which might give rise to a sensitization of the appropriate immune cells leading to an upregulation of immune markers. With respect to a difference in development between the innate and the adaptive component of immunity, the observation that the development of the latter starts at birth whereas that of innate immunity already during pregnancy [24] might be of importance here.