Lymphoid Organs Overview

Lymphoid Organs

• Lymphoid organs are aggregates of immune cells, specialized for protection. The primary function is defense against infections and tumors, and repair of dead cells and tissue.
• The immune system may abnormally attack normal cells causing inflammatory diseases like rheumatoid arthritis and systemic lupus erythematosus. This happens when immune system attacks self-antigens.
• Self-tolerance means immune system doesn't react against self-antigen.

Learning Objectives

• LO1: Importance of lymphoid organs - defense against infections and tumors, tissue repair
• LO2: Types of lymphoid organs
• LO3: Structure and function of thymus gland, role in T cell maturation and selection
• LO4: Structure and function of lymph nodes
• LO5: Structure and function of spleen
• LO6: Mucosa-associated lymphoid organs (MALT)
• LO7: HEVs and lymphocyte recirculation

Classification of Lymphoid Organs

• Primary Lymphoid Organs (Lymphocyte Differentiation):
  • Also called central lymphoid organs
  • Sites of immature lymphocyte development, proliferation, and maturation of stem cells
  • Includes thymus and bone marrow
• Secondary Lymphoid Organs (Lymphocyte Activation):
  • Sites where antigens are localized for effective exposure to mature lymphocytes.
  • Initiate adaptive immune responses.
  • Includes lymph nodes, tonsils, appendix, Peyer's patches, and mucosa-associated lymphoid tissue (MALT).

Thymus Gland

• Site: Superior mediastinum, behind the sternum.
• Shape: Bilobed. Present in children; atrophied in adults.
• Function:
  • Promotes T lymphocyte production and maturation.
  • Educates developing thymocytes through positive and negative selection.
  • Promotes central tolerance (prevents autoimmunity) by selectively removing T cells reactive against self-antigens.
• Structure:
  • Stroma (thin capsule, septa/trabeculae, epithelial cells)
  • Parenchyma (thymic lobules, cortex, medulla).

Thymic Lobule

• Contains cortex and medulla
• T-lymphocytes are in the cortex, mature T cells move to medulla.
• Inner cortex- squamous TECs, form cortico-medullary barrier.
• Outer cortex- stellate TECs, present self/non-self antigens to test efficiency, act as antigen-presenting cells.
• The cortex and medulla are crucial for T cell development.

Inner Medulla

• Mature T lymphocytes (fewer, less densely packed, larger and lightly stained)
• Medulla lacks blood thymic barrier; lymphocytes easily leave for the blood.
• Stellate TECs do not secrete cytokines.
• Hassall corpuscles (unique to medulla) support development of dendritic cells and the maturing of T cells.

Lymph Node Structure

• Site: Various locations throughout the body.
• Shape: Bean-shaped.
• Surfaces: Convex and concave.
• Structure:
  • Stroma (capsule, trabeculae, supporting reticular tissue)
  • Parenchyma (cortex, paracortex, medulla)
  • 3 distinct regions (cortex, paracortex, medulla)
  • Cortex has B cell zones, paracortex has T cell zones
  • Medulla has medullary cords, consisting of T cells, B cells, antibodies, macrophages, etc.

Lymph Node Function "Antigen Supply"

• Humoral Immune Response (B cells):
  • B cell activation leads to plasma cell production and distribution of antibodies.
• T cell activation and interaction:
  • T cells activated in the paracortex of the lymph node.
  • Lymph node acts as a filter for microbes.

High Endothelial Venules (HEVs)

• Major source of lymphocytes in lymph nodes (90%)
• Involved in lymphocyte recirculation; found in paracortex
• Allows lymphocytes to enter lymph nodes

Spleen

• Site: Left upper quadrant of the abdomen.
• Structure:
  • Stroma (capsule, trabeculae, supporting reticular tissue)
  • Parenchyma (white pulp, red pulp - responsible for the red color)
• Function:
  • Filtration of blood; removal of damaged red blood cells (RBCs); lymphocyte activation and antibody production.
  • Provides environment for B and T cell activation, antibody production, and removal of worn/damaged RBCs.

Spleen Vascular Supply and Function

• Closed circulation: Filtered blood communicates with spleen sinusoids for filtration and then returns to the vascular system
• Open circulation: Blood diffuses freely into the red pulp, allowing for filtering and processing of blood components.

Mucosa-Associated Lymphoid Tissue (MALT)

• Non-capsulated collections of lymphocytes, plasma cells, antigen-presenting cells (APCs) and lymphoid nodules.
• Located in respiratory, genitourinary, and gastrointestinal tracts.
• Dominant cell type is B cells, producing secretory IgA, targeting pathogens.

Peyer's Patches

• Specialized tissue in the ileum (part of the small intestine).
• Composed of lymphoid follicles and M cells (modified epithelial cells).
• Antigens pass through M cells to trigger immune responses.

Lymphocyte Recirculation

• Lymphocytes continuously circulate between blood, spleen, lymph nodes, and other lymphoid organs.
• Provides constant monitoring of the body and enhances exposure to foreign antigens.

T Cell Selection and Maturation

• Selection process educates T cells not to recognize/strongly bind to self-antigens.
• Positive Selection (Cortex): T cells test receptor efficiency; If they bind to self-antigen (negative outcome), they are marked for apoptosis (programmed cell death).
• Negative Selection (Medulla): Dendritic cells present self/non-self antigens; if they bind to self antigen (negative outcome) they die by apoptosis, otherwise they are kept.