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 only the first two lines of defense are ready to fight pathogens. These two lines of defense are the physical and chemical barriers (like skin and mucus), and the innate immune response (like inflammation and phagocytosis).

The third line of defense, which is the adaptive immune response, develops slowly over time for several reasons:

1. Exposure to Pathogens: The adaptive immune system relies on exposure to pathogens to develop. When a new pathogen enters the body, the immune system creates specific antibodies to fight it. Once the pathogen is defeated, some of these antibodies remain in the body, providing immunity against future infections from the same pathogen. Since newborns have not been exposed to many pathogens, their adaptive immune system is still undeveloped.

2. Maturation of Immune Cells: The cells responsible for the adaptive immune response, such as T cells and B cells, need time to mature. This maturation process involves the cells learning to recognize and respond to specific antigens without attacking the body's own cells. In newborns, this process is still ongoing.

3. Transfer of Antibodies: Newborns receive some adaptive immunity from their mother through the placenta and breast milk. These maternal antibodies provide temporary protection against some diseases, but they also suppress the baby's own immune response. This means the baby's adaptive immune system develops slowly, as the maternal antibodies gradually decrease.

4. Vaccination: Vaccines are a crucial part of developing the adaptive immune system. They expose the body to a harmless version of a pathogen, allowing the immune system to produce antibodies without causing disease. Since newborns have only started their vaccination schedule, their adaptive immunity is still developing.

In conclusion, the third line of defense in newborns develops slowly over time due to lack of exposure to pathogens, the need for immune cells to mature, the presence of maternal antibodies, and the ongoing process of vaccination.