Lymphatic System Histology

Lymphatic System Overview

• Composed of lymphocytes (B cells, T cells, natural killer cells) and antigen-presenting cells (dendritic cells, Langerhans' cells, macrophages).
• Includes lymphatic tissue, which is diffuse and nodular.
• Contains organs such as lymph nodes, spleen, and thymus, as well as lymphatic vessels that transport cells and fluid.

Classification of Lymphoid Organs

• Primary Lymphoid Organs:

  • Thymus, bone marrow, and lymphatic nodules in the distal intestinal tract (e.g., ileum, appendix).
  • Serve as "education centers" for lymphocytes, allowing differentiation into immunocompetent cells.
  • T cells mature in the thymus, while B cells mature in bone marrow and gut-associated lymphoid tissue.

• Secondary Lymphoid Organs/Tissue:

  • Include spleen, lymph nodes, mucosal-associated lymphoid tissue (MALT), and tonsils.
  • Act as sites for antigen-dependent activation and differentiation of immunocompetent lymphocytes into memory and effector cells.

Primary Lymphoid Organ Function

• Bone marrow and thymus initiate lymphocyte education; gut-associated lymphoid tissue contributes to this process.
• Lymphocytes develop the ability to distinguish "self" from "nonself" without the presence of antigens.
• Post-education, lymphocytes migrate through the bloodstream to populate mucosas, epidermis, connective tissues, and secondary lymphoid organs.

Secondary Lymphoid Organ Function

• Lymph nodes and associated structures are secondary training grounds for lymphocytes.
• Immunocompetent lymphocytes undergo antigen-dependent activation, leading to immune memory and effector cell production.
• The tight Blood-Thymus Barrier prevents other cells and free antigens from entering the thymus:
  • Composed of endothelial cell walls, perivascular connective tissue, and epithelioreticular cell layer.
  • T-cells can only enter the thymus via bloodstream; no afferent lymph vessels present.

Lymphatic Vessels and Metastasis

• Characterized by blind-ended vessels with high permeability to proteins and cells.
• Serve as pathways for potential cancer metastasis.

Germinal Center Formation

• Activated B-cells establish germinal centers in secondary follicles after encountering antigens and receiving T-cell assistance.
• B-lymphocytes proliferate into lymphoblasts, which further differentiate into plasma and memory cells or undergo apoptosis if aberrant.
• Germinal centers contain various lymphocyte types and are notable for active immune response maturation.

Key Locations of Lymphoid Tissue

• Mucosa-associated lymphoid tissue (MALT) is found in regions like the gastrointestinal, respiratory, and genitourinary tracts.
• Peyer’s patches consist of lymphoid follicles aggregated in the ileum, important for immune surveillance in the gut.