Which system of the body is primarily responsible for fighting off foreign substances?

The immune system is primarily responsible for fighting off foreign substances, such as pathogens (bacteria, viruses, fungi, and parasites) and other harmful invaders. It consists of various cells, tissues, and organs, including white blood cells, the lymphatic system, the spleen, and the thymus, all working together to detect and eliminate threats to the body.

When the immune system identifies a foreign substance, it activates an intricate response to neutralize and destroy it. This process can involve the production of antibodies, cellular responses, and the mobilization of various immune cells. The effectiveness of the immune system is crucial for maintaining health and preventing infections.

Considering the other options, the respiratory system plays a role in protecting against some pathogens (such as by filtering and trapping particles in the nasal passages and generating a cough reflex), but it is not primarily focused on attacking foreign substances comprehensively. The endocrine system regulates bodily functions through hormones but does not directly handle immune responses. An anaphylactoid reaction refers to a specific type of immune response, particularly related to allergic reactions, but does not represent a system itself. Hence, the immune system stands out as the main defense mechanism against foreign invaders.