Lymph nodes

The lymph node, abbreviated LN, is very important in pathology. It is the site of mets and may be the site of a primary lesion, i.e. lymphoma.

Haematopathology and lymphoma is dealt with in the haematopathology article. This article covers the basics of LNs.

LN architecture

Lymph node architecture (superficial to deep):

• Capsule.
• Subcapsular sinus.
  • Place where metastatic cells often are found.
• Cortex:
  • Follicles (superficial).
  • Paracortex (deep).
  • Cortical sinuses.
• Medulla:
  • Medullary cords.
  • Medullary sinuses.

Notes:

• Trabeculae - segment the LN radially, i.e. are spoke-like structures.

LN follicles

Types of follicles:

1. Primary follicle - no germinal center (pale area).
2. Secondary follicle - have a germinal center (pale area) and mantle zone (rim of darker staining lymphocytes).

Germinal centre

Architecture:

• Paler at capsular aspect.
• Darker at medullary aspect.

Location and cell types

Germinal centers:

• Site of B lymphocyte proliferation; follicle center cells.
  • Centrocytes transforming to immunoblasts (superficial aspect - paler).
  • Centrocytes (mid germinal center).
  • Centroblasts (deeper aspect - darker).
• Tingible body macrophages - mid germinal center.
• Follicular dendritic cells.

Paracortex:

• Site of T lymphocytes.
• Interdigitating dendritic cell.

Medullary cords (site of immunoglobulin synthesis):

• B lymphocytes.
• Plasma cells.

Mantle zone:

• Memory B cells.

B cell cycle

B cell cycle/maturation:

1. Centroblast (CB):
   • Appearance:
     • Large cells.
     • Round nucleus, dark.
     • Nucleolus (nucleoli).
     • Mitotically active.
   • Location:
     • Medullary aspect of germinal center.
2. Centrocytes (CC), AKA cleaved cells.
   • Appearance:
     • Cleaved nucleus
     • NOT mitotically active.
   • Location:
     • Mid germinal center.
3. Immunoblasts or memory B cells.
   • Appearance:
     • Small lymphocytes.
   • Location:
     • Peripheral blood, mantle zone.

Notes:

• Immunoblasts may be T cells or B cells; it is not possible to tell on morphologic grounds, i.e. without IHC.

Image:

• Lymphoid organs (healthsystem.virginia.edu).

Cell types

Follicular dendritic cells:

• Classically found in pairs.
• Large nucleus with:
  • Basketweave chromatin pattern.
  • Small central nucleolus.

Lymphocytes

• See B cell cycle section.

See also

• Haematopathology.
• Basics.

External links

• Lymph node architecture (umdnj.edu).