Cells and Organs of Immune System

Primary Lymphoid Organs - Immune Cell Nurseries

Primary Lymphoid Organs (PLOs) are sites where lymphocytes develop and mature from progenitor cells, becoming immunocompetent.

• Bone Marrow (BM)

  • Central site for hematopoiesis; origin of all immune cells (lymphoid and myeloid progenitors).
  • Maturation site for B-lymphocytes and Natural Killer (NK) cells.
  • Contains Hematopoietic Stem Cells (HSCs).

• Thymus

  • Bi-lobed organ in anterior superior mediastinum; crucial for T-cell maturation & selection.
  • Structure: Outer cortex (densely packed immature thymocytes, positive selection) and inner medulla (mature thymocytes, negative selection, Hassall's corpuscles).
  • Undergoes age-related involution (atrophy).
  • 📌 Selection Mnemonic: "Be Positive you can bind MHC in the Cortex; Be Negative about self-reactivity in the Medulla."

Comparison: Bone Marrow vs. Thymus

⭐ DiGeorge Syndrome (chromosome 22q11.2 deletion) is characterized by thymic aplasia/hypoplasia, leading to deficient T-cell mediated immunity and recurrent infections.

Secondary Lymphoid Organs - Battlegrounds & Hubs

• HEVs (High Endothelial Venules) in LNs facilitate lymphocyte entry.
• M-cells in MALT transport luminal antigens.
• ⭐ > Post-splenectomy: ↑ risk of sepsis from encapsulated bacteria (S. pneumoniae, H. influenzae, N. meningitidis). 📌 SHiN.

Innate Immune Cells - First Responders

Rapid, non-specific defense. Key players include:

-   **Mast Cells:** Release histamine; allergy, anaphylaxis.
-   **Eosinophils:** Combat parasitic infections; allergic reactions. Markers: MBP.
-   **Basophils:** Release histamine, heparin; allergic inflammation. Markers: CD203c.

⭐ Toll-Like Receptors (TLRs) on macrophages and dendritic cells recognize Pathogen-Associated Molecular Patterns (PAMPs), initiating innate immune responses.

Adaptive Immune Cells - Specialized Forces

• T-lymphocytes (T-cells): Cell-mediated immunity. Mature in Thymus.
  • Helper T (Th): CD4+. Orchestrate response. Subsets: Th1 (IFN-γ), Th2 (IL-4, IL-5), Th17 (IL-17).
  • Cytotoxic T (CTL): CD8+. Kill infected & tumor cells.
  • Regulatory T (Treg): CD4+, CD25+, FOXP3+. Suppress immunity, maintain tolerance.
  • 📌 Rule of 8: $CD4 \times MHCII = 8$; $CD8 \times MHCI = 8$.
• B-lymphocytes (B-cells): Humoral immunity. Mature in Bone marrow.
  • Differentiate: Plasma cells (Antibody factories: IgM, IgG, IgA, IgE) & Memory B-cells (rapid recall).
  • Markers: CD19, CD20, CD21.

⭐ IL-2, from activated T-cells, is vital for T-cell proliferation & differentiation.

and B-cell differentiation into plasma and memory cells)

High‑Yield Points - ⚡ Biggest Takeaways

• Primary lymphoid organs are Bone Marrow (B-cell maturation) and Thymus (T-cell maturation).
• Secondary lymphoid organs (Spleen, Lymph Nodes, MALT) are sites of antigen encounter and immune response initiation.
• T-cell education in the thymus involves positive selection (MHC restriction) and negative selection (self-tolerance).
• B-cell maturation in bone marrow involves negative selection to eliminate self-reactive B cells.
• CD4+ T-cells (Helper T cells) recognize MHC Class II; CD8+ T-cells (Cytotoxic T cells) recognize MHC Class I.
• Dendritic cells are the most potent professional Antigen Presenting Cells (APCs).
• Natural Killer (NK) cells mediate innate immunity against virally infected/tumor cells and perform ADCC (Antibody-Dependent Cell-mediated Cytotoxicity).