Final answer:
The capsule is the layer of dense connective tissue that covers the lymph nodes, spleen, and thymus, providing structure and aiding in immune responses.
Step-by-step explanation:
The capsule is a layer of dense connective tissue covering lymph nodes, the spleen, and the thymus. This connective tissue capsule is essential for providing structure and protection to these lymphoid organs. The capsule further divides these organs into lobules through extensions called trabeculae. In the lymph nodes and spleen, the capsule and trabeculae also support a network of reticular fibers, which provide additional structural support. The capsule is dense and fibrous, allowing these organs to effectively filter lymphatic fluid and blood, and facilitating an immune response to pathogens.
The spleen, a major secondary lymphoid organ, differs slightly in that it has a more fragile capsule, but it is still a vital component for immune functions along with the lymph nodes and thymus, which are encapsulated and structured for immune surveillance and response.