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Which organs are considered primary lymphoid organs?
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immune system
Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, TH1 and TH2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.
Primary lymphoid organs are the sites where lymphocytes—the cells responsible for immune responses—undergo maturation. In humans, the key primary lymphoid organs are the thymus and bone marrow.1
- Bone Marrow: This tissue serves as the origin for all lymphocytes, which develop from stem cells. B lymphocytes complete their maturation process within the bone marrow.1
- Thymus: Prospective T cells migrate from the bone marrow to the thymus to undergo further growth, differentiation, and maturation.12
In addition to these, the fetal liver is considered a primary lymphoid organ, and in birds, a structure known as the bursa of Fabricius also serves this function. Once mature, B and T lymphocytes exit these primary organs and travel to secondary lymphoid organs to encounter antigens and become activated.1