Final answer:
The principal lymphoid organs in the body include the primary organs: bone marrow and thymus gland; and secondary organs: spleen, lymph nodes, and tonsils. The primary organs are where lymphocytes mature and the secondary organs are where they are stored and activated to fight pathogens.
Step-by-step explanation:
The principal lymphoid organs in the body are both primary and secondary organs. The primary lymphoid organs are the bone marrow and the thymus gland, where lymphocytes are produced and mature. Specifically, B cells mature in the bone marrow, while T cells mature in the thymus. Secondary lymphoid organs include the spleen, lymph nodes, and tonsils. These structures do not produce lymphocytes but are crucial for initiating adaptive immune responses through the filtration of lymph and blood, storing lymphocytes, and exposing them to pathogens.
The spleen is the largest of the secondary organs and filters blood, removing unwanted red blood cells and pathogens, triggering the release of pathogen-fighting cells. Lymph nodes are distributed throughout the body and filter lymph as it passes through, acting as sites for pathogen encounter and lymphocyte activation. The tonsils, part of Waldeyer's ring, are located at the juncture of the respiratory and gastrointestinal tracts and are important for defending against pathogens entering through the mouth or nose.