Digested fats enter the lymphatic circulation through special capillary vessels called:

A. An antigen is any foreign substance (such as bacteria, viruses, or toxins) that triggers an immune response in the body. The immune system recognizes and responds to antigens, typically by producing antibodies.
B. Inflammation is a response of the body to injury or infection, characterized by redness, heat, swelling, and pain, but it is not a foreign substance itself.
C. An allergen is a specific type of antigen that causes an allergic reaction. While all allergens are antigens, not all antigens are allergens.
D. A lymphocyte is a type of white blood cell involved in the immune response, but it is not a foreign substance. Lymphocytes help recognize and respond to antigens.

A. The appendix is an organ that contains lymphoid tissue and plays a role in immune function, but it does not specifically refer to macrophages and monocytes.
B. The reticuloendothelial system (also known as the mononuclear phagocyte system) consists of macrophages and monocytes found throughout the body. These cells are part of the immune system and are responsible for phagocytosis, or the engulfing and digesting of pathogens, dead cells, and debris.
C. Peyer's patches are clusters of lymphoid tissue found in the small intestine that monitor and respond to pathogens in the digestive tract.
D. Tonsils are lymphoid tissues located in the throat, playing a role in immune defense, but they are not part of the reticuloendothelial system.