Immune System Organs: Primary Organs

Bone marrow—immune cell production, B cell maturation
Thymus—T cell maturation

Immune System Organs: Secondary Organs

Spleen, lymph nodes, tonsils, Peyer patches
Allow immune cells to interact with antigen

Lymph Node Overview

A 2° lymphoid organ that has many afferents, 1 or more efferents. Encapsulated, with trabeculae. Functions are nonspecific filtration by macrophages, circulation of B and T cells, and immune response activation.

Lymph Node Structure: Follicles and Medulla

Follicle

Located in outer cortex; site of B-cell localization and proliferation. 1° follicles are dense and quiescent. 2° follicles have pale central germinal centers and are active.

Medulla

Consists of medullary cords (closely packed lymphocytes and plasma cells) and medullary sinuses (contain reticular cells and macrophages). Medullary sinuses communicate with efferent lymphatics.

Lymph Node Structure: Paracortex and Clinical Relevance

Paracortex

Contains T cells. Region of cortex between follicles and medulla. Contains high endothelial venules through which T and B cells enter from blood. Underdeveloped in patients with DiGeorge syndrome.

Paracortex enlarges in an extreme cellular immune response (e.g., EBV and other viral infections → paracortical hyperplasia → lymphadenopathy).

Lymph Node Histology

#boundingbox_A

#boundingbox_B

Images A and B display histological sections of a lymph node, revealing the dense cellular organization. In Image A, on the left, you can observe multiple distinct follicles within the cortex. Image B, on the right, provides another view, also showing follicles and the surrounding tissue. These microscopic views help us visualize the structural components, such as the follicles and the paracortex, that we’ve discussed, highlighting the cellular architecture that supports immune function.

Lymph Node Diagram

#boundingbox_diagram

This detailed diagram illustrates the internal structure of a lymph node, showing the flow of lymph and the arrangement of its components. Notice the afferent lymphatic vessels bringing lymph into the capsule, which then filters through the cortex, paracortex, and medulla before exiting via the efferent lymphatic vessel. Key structures like follicles (1° and 2°), germinal centers, mantle zone, medullary cords, and sinuses are clearly labeled, along with the blood supply including arteries, veins, and postcapillary venules. The trabeculae, which are extensions of the capsule, are also visible.

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