What is the main difference between natural immunity and acquired immunity?

Natural immunity refers to the defense mechanisms that a person is born with, which include physical barriers like skin and mucous membranes, as well as innate immune responses that are always present. This type of immunity operates immediately upon exposure to pathogens. On the other hand, acquired immunity, also known as adaptive immunity, develops over time as a response to exposure to specific pathogens or through vaccinations. This means that individuals gain acquired immunity by having the immune system recognize and remember specific antigens after exposure, allowing for a more robust response during subsequent encounters with the same pathogen.

Thus, the correct answer highlights that natural immunity is inherent from birth, while acquired immunity is not present at birth and instead emerges through experiences with various pathogens or through vaccination, making it a crucial distinction in immunology.