Lymphoid Organs and Immune System

Immune Cells & Basics - Tiny Troopers Team-Up

• Lymphocytes: Key players in adaptive immunity; morphologically small, round cells with scant cytoplasm.
  • T cells (Thymus-derived): Cell-mediated. All express CD3.
    • Helper (CD4+): Orchestrate immune response.
    • Cytotoxic (CD8+): Directly kill infected/tumor cells.
  • B cells (Bone marrow-derived): Humoral immunity; produce antibodies. Express CD19, CD20.
  • NK cells (Natural Killer): Innate immunity. Large granular lymphocytes.
• Macrophages: Phagocytosis, antigen presentation. From monocytes. Abundant cytoplasm, kidney-shaped nucleus.
• Dendritic Cells (DCs): Most potent APCs; link innate & adaptive. Star-shaped processes.
• Immunity Overview:
  • Innate: Rapid, non-specific first line (e.g., NK cells, macrophages).
  • Adaptive: Slower, specific, develops memory (e.g., T/B cells).

⭐ Key CD markers: T-cells are universally CD3+. Helper T-cells are CD4+. Cytotoxic T-cells are CD8+. B-cells are CD19+ and CD20+.

Primary Lymphoid Organs - Boot Camp & College

• Bone Marrow:
  • Central hematopoietic organ; origin of all blood cells.
  • Site for B lymphocyte maturation and development (antigen-independent phase).
  • Self-reactive B cells eliminated via negative selection.
• Thymus:
  • Bilobed organ in superior mediastinum; crucial for T lymphocyte maturation ("Thymic Education").
  • Blood-Thymus Barrier: In cortex; protects developing thymocytes from circulating antigens. Formed by epithelial reticular cells, capillary endothelium, and macrophages.
  • Hassall's Corpuscles: Found in medulla. Eosinophilic, concentric lamellated structures of Type VI epithelial reticular cells. 📌 Mnemonic: "Hassall's Help T-regs Survive."

⭐ Hassall's corpuscles are unique to the thymic medulla and are believed to play a role in inducing T regulatory (Treg) cell development, possibly by secreting cytokines like TSLP (Thymic Stromal Lymphopoietin).

Thymus: Cortex vs. Medulla

Secondary Lymphoid Organs I - Nodes & Spleen Scrutiny

Lymph Node: Filters lymph; initiates adaptive immunity.

• Structure:
  • Capsule: Dense CT.
  • Cortex: B-cell zone. Follicles (primary/secondary with germinal centers).
  • Paracortex: T-cell zone. High Endothelial Venules (HEVs) for lymphocyte entry.
  • Medulla: Medullary cords (plasma cells, macrophages) & sinuses.
• Lymph Flow:

Spleen: Filters blood; removes old RBCs; immune surveillance.

• Structure:
  • Capsule: With trabeculae.
  • White Pulp (Immune Response):
    • Periarteriolar Lymphoid Sheaths (PALS): T-cells around central artery.
    • Follicles: B-cells (may show germinal centers).
  • Red Pulp (Blood Filtration):
    • Splenic Cords (of Billroth): Macrophages, plasma cells.
    • Sinusoids: Discontinuous endothelium.
  • Marginal Zone: Interface between red/white pulp; antigen trapping & presentation.

⭐ Key Cell Locations:

• Lymph Node: B cells primarily in follicles (cortex); T cells primarily in paracortex.
• Spleen: B cells in follicles (white pulp); T cells in PALS (white pulp).

Secondary Lymphoid Organs II - MALT & Mucosal Guards

• MALT (Mucosa-Associated Lymphoid Tissue):

  • Diffuse, unencapsulated lymphoid aggregates in submucosa (respiratory, GI, GU tracts).
  • Primary site of antigen encounter; initiates mucosal immunity.

• GALT (Gut-Associated Lymphoid Tissue):

  • Peyer's Patches: Prominent in ileum.
    • Contain specialized M (microfold) cells for antigen sampling.
    • Dome area rich in B-lymphocytes, APCs; germinal centers present.

  ⭐ > Peyer's patches are characterized by M cells, specialized epithelial cells that transport luminal antigens to underlying immune cells.

• Tonsils (Waldeyer's Ring): Guard pharynx.

  • 📌 Epithelium: Palatine/Lingual - Stratified Squamous; Pharyngeal - Pseudostratified Columnar.

  • Comparison of Tonsil Types:

    Feature	Palatine Tonsil	Pharyngeal Tonsil (Adenoids)	Lingual Tonsil
    Epithelium	Stratified squamous (non-keratinized)	Ciliated pseudostratified columnar	Stratified squamous (non-keratinized)
    Crypts	10-20 deep, branched	Folds/pleats (no true crypts)	Single, short crypt per unit
    Capsule	Partial, thick	Thin, incomplete	Thin, poorly defined

High‑Yield Points - ⚡ Biggest Takeaways

• Thymus: Essential for T-cell maturation; characterized by Hassall's corpuscles.
• Spleen: White pulp for immune reactions; red pulp for blood filtration.
• Lymph Nodes: Paracortex is T-cell rich; germinal centers in follicles indicate B-cell activation.
• MALT: Includes Peyer's patches (ileum) for mucosal immunity.
• Primary lymphoid organs (Bone Marrow, Thymus) are sites of lymphocyte development.
• Secondary lymphoid organs (Spleen, Lymph Nodes) initiate adaptive immune responses.
• DiGeorge Syndrome: Results from thymic aplasia, leading to severe T-cell deficiency.