In newborn babies, only the first two lines of defence are ready to fight pathogens.Explain why the third line of defence develops slowly over time.

The immune system of a newborn baby is not fully developed at birth, which is why it relies heavily on the first two lines of defense: the physical and chemical barriers (like skin and mucus), and the innate immune response (like inflammation and phagocytosis).

The third line of defense, also known as the adaptive immune response, is more complex and specific. It involves the production of specific antibodies in response to specific pathogens. This system is based on the ability of the immune cells to "remember" previous encounters with pathogens, which allows for a faster and more effective response in case of a second encounter.

However, this "memory" takes time to develop. The adaptive immune system needs exposure to different pathogens to learn and remember them. This is why newborns are more susceptible to infections, as their adaptive immune system is still immature.

Over time, as the baby is exposed to various pathogens, the adaptive immune system matures and becomes more efficient in fighting off infections. This is also why vaccinations are important, as they help to "train" the adaptive immune system by exposing it to harmless versions of pathogens, thus helping it to develop its "memory".

In summary, the third line of defense in newborns develops slowly over time because it requires exposure to various pathogens to mature and become more efficient.