🔬 Lymphatic System Lesson

CPM: three zones inside a lymph node

A lymph node isn't a uniform blob — it has distinct internal zones, each housing a different type of immune cell doing a different job.

Cortex

Paracortex

Medulla

📖 Full Breakdown

Lymph flows in, gets filtered zone by zone, then flows out

Lymph enters through afferent vessels, percolates through all three zones, then exits via efferent vessels.

Cortex (outer)

B-cell territory

Contains lymphoid follicles packed with B cells. During an active immune response, germinal centers form here as B cells proliferate.

Paracortex (middle)

T-cell territory

Also contains dendritic cells, which present antigens to T cells here — this is where antigen presentation to T cells specifically takes place.

Medulla (inner)

Macrophages and plasma cells

Houses macrophages performing final filtration and plasma cells actively secreting antibodies — the last stop before filtered lymph exits the node.

🩺 Clinical / Exam Application

A patient with an infection develops noticeably swollen, tender lymph nodes near the infection site. This lymphadenopathy happens because all three zones of affected lymph nodes are working overtime simultaneously: the cortex is rapidly producing B cells in expanding germinal centers, the paracortex is presenting antigens to activate T cells, and the medulla is churning out antibody-secreting plasma cells — the physical swelling reflects this genuine, coordinated increase in immune cell activity, not just passive fluid buildup.

⚠️ Exam Alert

Sentinel lymph node biopsy — a frequently tested clinical application — relies on the fact that lymph node structure allows pathologists to examine the FIRST node draining a tumor site for cancer spread, since lymph flows through nodes in a predictable, sequential pattern from a given body region.

🚧 Common Trap

Don't confuse the cortex (B cells) with the paracortex (T cells) just because both are described as "outer" regions relative to the medulla. The cortex is the outermost zone with B cells in follicles; the paracortex sits between the cortex and medulla and contains T cells — they are distinct middle layers, not interchangeable terms for the same zone.

✅ Quick Check

During an active infection, which zone of the lymph node would show the most dramatic increase in size due to forming germinal centers, and why?

📝 Exam Prep

Common Exam Questions

❓ What happens in the lymph node during an immune response?

✅ Antigens arrive via afferent lymphatics. Antigen-presenting cells present antigens to T cells in the paracortex. B cells in the cortex are activated and form germinal centers, proliferating and differentiating into plasma cells that produce antibodies.

❓ What are the three zones of a lymph node and what does each contain?

✅ The cortex (outer) contains B cells in lymphoid follicles. The paracortex (middle) contains T cells and dendritic cells. The medulla (inner) contains macrophages and antibody-secreting plasma cells.