Final answer:
Plasmodium falciparum is the pathogen that uses antigenic variation to evade immune defenses, by changing its surface proteins and thus avoiding recognition by the host's immune system.
Step-by-step explanation:
The pathogen that undergoes antigenic variation to avoid immune defenses is Plasmodium falciparum. This organism is responsible for causing malaria and evades the host immune system by altering its surface proteins with each generation. Antigenic variation allows P. falciparum to persist in the host by staying one step ahead of the immune response, which is otherwise primed to recognize and attack pathogens based on the antigens they present. This evasion is similar to what happens with bacteria like Neisseria gonorrhoeae and Borrelia burgdorferi, which also undergo antigenic variation to avoid being targeted by the host's immune system.