Which of the following are part of the first line of defense against invaders: (select all that apply)

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.

A. Lymph nodes are secondary lymphoid organs where B and T cells can be activated, but they are not the cells themselves.
B. Tonsils are lymphoid tissues that help protect against pathogens, but they are not the cells themselves.
C. Lymphocytes are the correct answer. B cells and T cells are types of lymphocytes, which are white blood cells involved in the adaptive immune response.
D. Antibodies are proteins produced by B cells (specifically plasma cells) in response to pathogens, but they are not cells themselves.