U01.02.001 Immune system organs

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Learning Objectives

Master the structural and functional organization of the Immune System Organs. Distinguish between primary and secondary lymphoid organs, and understand the internal anatomy of the lymph node—specifically the cortex, paracortex, and medulla—for the USMLE Step 1.

1. Primary Lymphoid Organs

Primary ( $1^\circ$ ) lymphoid organs are the sites of immune cell production and maturation.

Organ	Function	Key Cells
Bone Marrow	Hematopoiesis (production of all immune cells).	B-cell maturation occurs here.
Thymus	Site of T cell selection and differentiation.	T cell maturation occurs here.

2. Secondary Lymphoid Organs

Secondary ( $2^\circ$ ) lymphoid organs are the sites where mature immune cells interact with antigens to initiate an adaptive immune response.

Organ	Function
Spleen	Filters blood; site of immune response to blood-borne antigens.
Lymph Nodes	Filters lymph; nonspecific filtration by macrophages and T/B cell activation.
MALT / GALT	Tonsils, adenoids, appendix, and Peyer patches. Protects mucosal surfaces.

3. Anatomy of the Lymph Node

The lymph node consists of specific zones designated for B cells, T cells, and filtration.

Region	Cell Type & Characteristics	Clinical Note
Follicle (Cortex)	B cells. 2º follicles have pale germinal centers (active proliferation).	Absent in X-linked agammaglobulinemia.
Paracortex	T cells. Contains High Endothelial Venules (HEV) for lymphocyte entry.	Underdeveloped in DiGeorge Syndrome; enlarged in viral infections (EBV).
Medulla	Cords (plasma cells) and Sinuses (macrophages/reticular cells).	Sinuses lead to efferent lymphatics.

Activity:

High-Yield Clinical Pearls:

Paracortical Hyperplasia: In cellular immune responses (like Mononucleosis), the paracortex expands, causing visible lymphadenopathy.
DiGeorge Syndrome: Patients lack a thymus, leading to a profound T-cell deficiency and a visible lack of the paracortex in lymph nodes.
Post-Splenectomy: Since the spleen is a secondary lymphoid organ, its removal increases susceptibility to encapsulated organisms (S. pneumoniae, H. influenzae, N. meningitidis).