Final answer:
Diffuse lymphatic tissue is a type of secondary lymphoid organ that is not encapsulated and helps defend against pathogens in the respiratory and digestive tracts.
Step-by-step explanation:
The secondary lymphoid organ, diffuse lymphatic tissue, refers to a type of lymphatic tissue that is not encapsulated like the spleen or lymph nodes.
It consists of areas of lymphoid tissue that are found underlying epithelial membranes, such as the respiratory and digestive tracts.
These areas are exposed to environmental pathogens, and the diffuse lymphoid tissue helps defend against these pathogens by containing a dense cluster of lymphocytes.