What is the primary purpose of the tonsils in the immune system?

The primary purpose of the tonsils within the immune system is to trap pathogens that enter the body through the mouth and throat. Tonsils are lymphoid tissues located in the pharyngeal region, and they serve as a first line of defense against infections. When pathogens such as bacteria or viruses enter through the oral cavity, the tonsils identify and help to eliminate these harmful agents by producing immune responses. They contain lymphocytes, which play a vital role in recognizing and responding to foreign invaders, thus contributing to the immune system's overall function.

The other options do not accurately describe the role of the tonsils. They do not produce hormones, filter air directly, or regulate blood flow—functions associated with other organs and systems in the body. Therefore, the ability of the tonsils to trap pathogens is key to their function within the immune system, making it the correct and most relevant choice.