What type of immunity is produced by the immune system in response to an antigen?

Active immunity is generated when the immune system actively responds to an antigen, leading to the production of antibodies and memory cells specific to that antigen. This process occurs when the body encounters a pathogen, such as a virus or bacterium, for the first time, or through vaccination, where a harmless form of the antigen is introduced to stimulate the immune response.

The key aspect of active immunity is that it provides long-term protection; once the individual has been exposed to an antigen and the immune system has responded, it retains a 'memory' of the pathogen, enabling a faster and more robust response upon subsequent exposures. This is in contrast to other types of immunity, which do not involve the activation of the immune system in the same way. For instance, passive immunity involves the transfer of antibodies from one person to another (like from mother to child) and does not involve the recipient's immune system generating a response. Innate immunity refers to the body's first line of defense mechanisms that act broadly and immediately upon infection, but it does not create a specific immune memory. Acute immunity is not a recognized category of immune response. Thus, active immunity is the correct answer in this context, as it specifically refers to the direct response of the immune system to an antigen