What is an example of a pathogenic organism that induces an immune response?

An example of a pathogenic organism that induces an immune response is protozoa. Protozoa are single-celled eukaryotic organisms that can cause diseases in humans and animals, such as malaria, giardiasis, and amoebic dysentery. When they invade a host, protozoa trigger the immune system to respond through various mechanisms, including the production of antibodies and activation of T-cells. This immune response is crucial for controlling and eliminating the infection.

The other options, while related to immune processes, do not represent pathogenic organisms. Antigens are molecules that provoke an immune response but are not organisms themselves; they can come from various sources, including bacteria, viruses, or parasites. HLA proteins are involved in the regulation of the immune response as they present antigens to T-cells but do not directly induce an immune response as pathogens do. Antibody fragments are products of the immune response, not pathogens that trigger it. Thus, protozoa are the correct choice as they directly represent pathogenic organisms inducing an immune response.