How do lymph nodes form?

Lymph nodes form through a process that begins during fetal development. Here's a step-by-step explanation of how they develop:

Embryological Development of Lymph Nodes

1. Mesenchymal Condensation: Lymph nodes start to develop in utero as mesenchymal condensations, which are aggregations of mesenchymal cells. These condensations eventually bulge to form lymph sacs.

2. Formation of Lymph Sacs: By around the 6th to 7th week of gestation, lymph sacs begin to form. These sacs are the precursors to the lymphatic system, including lymph nodes.

3. Development of T-Cell Regions: The T-cell regions within lymph nodes start developing around the 13th gestational week. This is when the paracortical area, rich in T cells, begins to take shape.

4. Development of B-Cell Regions: B-cell regions, such as the marginal sinus and outer cortex, start developing around the 14th gestational week. This is where B cells and their precursors begin to accumulate.

5. Maturation and Organization: By the 17th to 20th gestational weeks, the lymph nodes become more organized with distinct regions for T and B cells. The interdigitating reticulum cells, a subtype of dendritic cells, are found in the paracortical region, and primary follicles start to form in the outer cortex.

Postnatal Development and Function

After birth, lymph nodes continue to mature and become functional parts of the immune system. They filter lymph fluid, trap pathogens, and facilitate the activation of immune cells like T and B lymphocytes14.

Lymph nodes are crucial for the immune response, acting as filters for lymph fluid and sites for immune cell activation. Their structure includes a capsule, cortex, paracortex, and medulla, each playing a specific role in immune function12.