The presence of which of the following indicates an ongoing immune response?

The presence of IgG antibodies indicates an ongoing immune response because these antibodies play a crucial role in the body's adaptive immunity. When the immune system is first exposed to a pathogen, it primarily produces IgM antibodies, which are the first line of defense. As the immune response matures, the body starts to produce IgG antibodies. These are more specific to the pathogen and are produced in larger quantities to effectively neutralize or eliminate the invader.

IgG antibodies have a longer lifespan compared to IgM and are crucial for providing lasting immunity. Their presence in the bloodstream generally indicates that the host is currently fighting an infection or has previously been exposed to an antigen, and thus the immune system is actively responding.

Other components such as memory B-cells and interleukins do play roles in immune responses but are not direct indicators of an active response to an ongoing infection. Memory B-cells, for example, are responsible for the rapid response to subsequent infections by the same pathogen, while interleukins serve as signaling molecules that help modulate the immune response but do not directly indicate current activity. Therefore, the presence of IgG antibodies is the most definitive sign of an ongoing immune response.