Any foreign substance that enters the body an induces an immune response is called a(n):

A. Interferon is a type of protein produced by the body in response to infections, particularly viral infections. It is not related to allergens.
B. Macrophage is a type of white blood cell that engulfs and digests pathogens, dead cells, and other debris. It is not an allergen.
C. An allergen is a substance that triggers an allergic reaction. It is considered a type of antigen—a foreign substance that the immune system recognizes and reacts to, often causing symptoms like inflammation, swelling, or itching in allergic individuals.
D. Antibody is a protein produced by B cells that binds to specific antigens, but it is not an allergen.

A. Blood contains various types of cells, such as red blood cells and white blood cells, but dust cells are not found in the blood.
B. Dust cells are a type of macrophage found in the lungs. They are responsible for engulfing and clearing inhaled particles, dust, and pathogens from the respiratory system.
C. The liver contains Kupffer cells, which are macrophages that filter blood, but dust cells are not found in the liver.
D. The small intestine contains various immune cells, but dust cells are not among them. The small intestine has its own specialized immune cells like Peyer's patches.