Which of the following is NOT a key component of the human immune system?

The respiratory system is not considered a key component of the human immune system. The immune system primarily includes specialized cells, proteins, and organs that work together to defend against pathogens.

Antibodies are crucial proteins produced by B cells that identify and neutralize foreign invaders like bacteria and viruses. White blood cells, which include various types such as lymphocytes and phagocytes, play a significant role in recognizing and responding to these pathogens. The lymphatic system is also a vital component of the immune response, as it helps transport lymph fluid, which contains immune cells, throughout the body and plays a role in filtering out pathogens.

In contrast, while the respiratory system has mechanisms to prevent infections (like mucous membranes and cilia), its primary function is to facilitate gas exchange. It helps bring oxygen into the body and remove carbon dioxide but does not directly contribute to the immune response in the same way that the other components do. Therefore, the respiratory system is not classified as a key part of the immune system.