Which type of immunity is acquired through direct infection and lasts the longest?

Natural active acquired immunity is the correct answer because it results from the body’s own immune response to a pathogen after infection. When an individual becomes infected with a virus or bacterium, the immune system activates and produces antibodies specifically targeting that pathogen. This process not only helps to clear the infection but also facilitates the formation of memory cells, which 'remember' the pathogen. If the person is exposed to the same pathogen in the future, these memory cells enable a quicker and more efficient immune response.

This form of immunity tends to last a significant length of time, sometimes even a lifetime, depending on the pathogen and the individual’s immune response. In contrast, innate immunity is non-specific and does not have memory capabilities. Artificial active acquired immunity, while also beneficial (often resulting from vaccinations), typically does not provide immunity as long-lasting as that acquired through natural infection. Finally, artificial passive acquired immunity involves the transfer of pre-formed antibodies to an individual (like in the case of immunoglobulin therapy), which provides short-term protection but does not lead to the development of memory cells, thus lacking long-term immunity.