Cells and Tissues of the Immune System

Primary Lymphoid Organs - Cell University

• Sites of lymphocyte development & maturation from stem cells.
• Bone Marrow:
  • Central hematopoietic organ: Origin of all blood cells.
  • B-lymphocyte (B-cell) maturation site.
  • B-cells acquire B-Cell Receptors (BCR); negative selection occurs.
• Thymus:
  • Bilobed organ; T-lymphocyte (T-cell) maturation & "education".
  • Cortex: Positive selection. T-cells must bind self-MHC. Failure -> apoptosis. Immature thymocytes (CD4+CD8+, double positive).
  • Medulla: Negative selection. T-cells binding self-antigens too strongly -> apoptosis (central tolerance). AIRE gene crucial. Mature thymocytes (CD4+ or CD8+, single positive). Hassall's corpuscles present.

⭐ DiGeorge Syndrome (22q11.2 deletion) leads to thymic aplasia/hypoplasia, causing T-cell deficiency, hypocalcemia (parathyroid hypoplasia), and cardiac defects. 📌 CATCH-22: Cardiac defects, Abnormal facies, Thymic hypoplasia, Cleft palate, Hypocalcemia/Hypoparathyroidism due to 22q11 deletion.

Secondary Lymphoid Organs & MALT - Antigen Arenas

• Lymph Nodes: Bean-shaped; filter lymph; initiate adaptive immunity.
  • Cortex: B-cell follicles (primary/secondary germinal centers).
  • Paracortex: T-cells, dendritic cells.
  • Medulla: Plasma cells, macrophages.
• Spleen: Filters blood; removes old RBCs, pathogens.
  • White Pulp: Periarteriolar lymphoid sheaths (PALS - T-cells), follicles (B-cells).
  • Red Pulp: Sinusoids, macrophages; RBC destruction.
• MALT (Mucosa-Associated Lymphoid Tissue): Protects mucosal surfaces.
  • Includes GALT (e.g., Peyer’s patches in ileum), BALT, NALT.
  • Site of IgA production.

⭐ Splenectomy significantly increases susceptibility to encapsulated bacteria (e.g., S. pneumoniae, H. influenzae type b, N. meningitidis).

Innate Immune Cells - First Responders

• Neutrophils (Polymorphonuclear Leukocytes, PMNs):
  • Most abundant phagocytes; key in acute inflammation.
  • Multi-lobed nucleus; granules contain bactericidal enzymes.
• Macrophages:
  • Phagocytosis; antigen presentation to T cells; cytokine production.
  • Derived from blood monocytes; reside in tissues (e.g., Kupffer cells, alveolar macrophages).
• Natural Killer (NK) Cells:
  • Lymphocytes; kill virus-infected cells & tumor cells without prior sensitization.
  • Recognize cells lacking MHC class I molecules.
• Dendritic Cells (DCs):
  • Most potent antigen-presenting cells (APCs).
  • Initiate adaptive immune responses; link innate and adaptive immunity.
• Mast Cells:
  • Located in connective tissues, near blood vessels.
  • Release histamine, leukotrienes, prostaglandins during allergic reactions & inflammation.
• Eosinophils:
  • Combat parasitic helminth infections; modulate allergic inflammatory responses.
  • Granules contain major basic protein.
• Basophils:
  • Circulating granulocytes; similar functions to mast cells.
  • Release histamine and other mediators in allergic responses.

⭐ Chronic Granulomatous Disease (CGD) is caused by a defect in NADPH oxidase, leading to recurrent infections with catalase-positive organisms (e.g., S. aureus, Aspergillus).

Adaptive Immune Cells - Elite Squad

• T-Lymphocytes (Cell-Mediated Immunity): Mature in Thymus.
  • Helper T (Th) cells: CD4+. Activate B-cells, Tc cells, macrophages. 📌 MHC-II restricted (CD4 x 2 = 8).
  • Cytotoxic T (Tc) cells: CD8+. Kill virus-infected & tumor cells. 📌 MHC-I restricted (CD8 x 1 = 8).
  • Regulatory T (Treg) cells: CD4+, CD25+. Suppress immune response, maintain tolerance.
• B-Lymphocytes (Humoral Immunity): Mature in Bone marrow.
  • Differentiate into Plasma cells (secrete antibodies) & Memory B-cells (long-term immunity).
  • Recognize antigens via B-cell receptors (BCR); require Th cell help for activation (most antigens).

⭐ CD4+ T-cell count is a critical marker for HIV infection progression; a count below 200 cells/μL defines AIDS.

High‑Yield Points - ⚡ Biggest Takeaways

• Primary lymphoid organs: Bone marrow (B-cell maturation) and Thymus (T-cell maturation).
• Secondary lymphoid organs (spleen, lymph nodes, MALT) are sites of antigen encounter and response initiation.
• CD4+ T-cells (helper) recognize MHC-II; CD8+ T-cells (cytotoxic) recognize MHC-I.
• Key B-cell markers include CD19, CD20; NK cells express CD16/CD56 and lack CD3.
• Dendritic cells are the most potent Antigen Presenting Cells (APCs), linking innate and adaptive immunity.
• Thymic negative selection is crucial for self-tolerance, eliminating self-reactive T-cells in the thymus medulla.
• Hassall's corpuscles are characteristic of the thymic medulla; their exact function is still debated but linked to T-reg development.