Immune System Practice Test

Lymphocytes concentrate in lymphatic tissue because these organs provide the dense, organized environment where immune cells meet antigens, interact with other immune cells, and proliferate to mount responses. Mature lymphocytes circulate through blood and lymph, but the highest concentrations sit in lymphoid organs like lymph nodes, the thymus, and the spleen. Lymph nodes trap lymph and present antigens to B and T cells, the thymus is where T cells mature and are selected, and the spleen’s white pulp is rich in lymphocytes that monitor blood-borne pathogens. In contrast, blood carries them, but not in high concentration; bone marrow is where they originate (and B cells mature), and the digestive tract has mucosa-associated lymphoid tissue but is not the main hub for lymphocyte concentration.