Which of the following is true about immunity?

The assertion that immunity can be acquired passively or actively is accurate and reflects fundamental immunology principles. Active immunity occurs when the body is exposed to a pathogen and produces its own antibodies in response, such as through natural infection or vaccination. In contrast, passive immunity involves the transfer of antibodies from another source to provide immediate protection without the host's direct immune response, exemplified by maternal antibodies passed to an infant through breast milk or intravenous immunoglobulin therapy.

This duality in the ways immunity can be obtained underlines the complexity of the immune system and its adaptability in protecting against diseases. The other options present more limited or incorrect viewpoints: immunity is not solely reliant on vaccination, does not equate to mere resistance, and while age can impact the immune system's effectiveness, the concept of immunity itself remains distinct and multifaceted.