Enumerate the cells of lymphoreticular system. Schematically represent the process of B-cell maturation.

Cells of the Lymphoreticular System

The lymphoreticular system is composed of various cell types, primarily lymphocytes and phagocytic cells, along with supporting stromal cells. These cells originate from hematopoietic stem cells in the bone marrow and undergo differentiation and maturation in primary and secondary lymphoid organs. The key cells of the lymphoreticular system include:

• Lymphocytes: These are the main functional cells of the adaptive immune system.
  • B Lymphocytes (B cells): Responsible for humoral immunity, they produce antibodies. They mature primarily in the bone marrow.
  • T Lymphocytes (T cells): Responsible for cell-mediated immunity. They mature in the thymus. T cells include:
    • Helper T cells (Th cells): Coordinate immune responses.
    • Cytotoxic T cells (Tc cells): Directly kill infected or cancerous cells.
    • Regulatory T cells (Treg cells): Suppress immune responses to maintain tolerance.
  • Natural Killer (NK) Cells: Part of the innate immune system, they kill virus-infected cells and tumor cells without prior sensitization.
• Phagocytic Cells: These cells engulf and digest pathogens and cellular debris.
  • Macrophages: Derived from monocytes, they are powerful phagocytes found in tissues. They also act as antigen-presenting cells (APCs).
  • Dendritic Cells (DCs): Highly effective APCs that present antigens to T cells, initiating adaptive immune responses. They are found in lymphoid organs and peripheral tissues.
• Other Supporting Cells:
  • Reticular Cells: Form the reticular fiber network in lymphoid organs, providing structural support and microenvironments for lymphocyte development.
  • Stromal Cells: Found in bone marrow and lymphoid organs, they provide growth factors and direct cell-cell interactions crucial for lymphocyte development and function.

Schematic Representation of B-cell Maturation

B-cell maturation is a complex, multi-stage process occurring primarily in the bone marrow (antigen-independent phase) and subsequently in secondary lymphoid organs (antigen-dependent phase). This process ensures the generation of diverse and functional B cells capable of producing antibodies.

The stages of B-cell maturation can be schematically represented as follows:

Stage	Location	Key Events & Characteristics	Surface Markers (Key)
1. Hematopoietic Stem Cell (HSC)	Bone Marrow	Pluripotent stem cell, origin of all blood cells.	CD34+
2. Common Lymphoid Progenitor (CLP)	Bone Marrow	Committed to lymphoid lineage (B, T, NK cells).	CD34+, CD10+
3. Pro-B Cell (Progenitor B cell)	Bone Marrow	Heavy chain D-J recombination begins. Supported by IL-7 from stromal cells.	CD19+, CD10+, CD45R+, C-kit+, CD43+
4. Pre-B Cell (Precursor B cell)	Bone Marrow	Complete heavy chain VDJ recombination. Expresses pre-B cell receptor (pre-BCR) with surrogate light chains. Light chain V-J recombination begins.	CD19+, CD10+, CD25+ (IL-2Rα), μ heavy chain in cytoplasm
5. Immature B Cell	Bone Marrow	Successful light chain V-J recombination. Expresses complete IgM on cell surface (BCR). Undergoes negative selection (tolerance induction).	CD19+, surface IgM (sIgM)
6. Transitional B Cell (T1/T2)	Bone Marrow to Spleen	Leaves bone marrow, migrates to spleen. Undergoes further selection and maturation cues.	sIgM high, sIgD low (T1); sIgM intermediate, sIgD intermediate (T2)
7. Mature Naïve B Cell	Secondary Lymphoid Organs (Spleen, Lymph Nodes)	Co-expresses IgM and IgD on the cell surface. Ready to encounter antigen.	CD19+, sIgM+, sIgD+
8. Activated B Cell	Secondary Lymphoid Organs (Follicles)	Encounters antigen, often with T-cell help. Proliferation and differentiation initiated.	Antigen-specific BCR engagement
9. Plasma Cell	Lymphoid Organs, Bone Marrow	Differentiates into antibody-secreting factories. Short-lived or long-lived.	High cytoplasmic Ig, low/no sIg, CD138+
10. Memory B Cell	Lymphoid Organs, Circulation	Long-lived cells that provide rapid and robust secondary immune response upon re-exposure to antigen.	Class-switched Ig (e.g., IgG, IgA, IgE), sIgM/sIgD (depending on type)

Simplified Schematic Representation:

Hematopoietic Stem Cell (Bone Marrow)
↓
Common Lymphoid Progenitor (Bone Marrow)
↓
Pro-B Cell (Heavy Chain Recombination)
↓
Pre-B Cell (Pre-BCR expression, Light Chain Recombination begins)
↓
Immature B Cell (Surface IgM, Negative Selection)
↓ (Migration to Spleen)
Transitional B Cell (T1/T2)
↓ (Maturation in Spleen)
Mature Naïve B Cell (Surface IgM & IgD, enters secondary lymphoid organs)
↓ (Antigen Encounter, T-cell help in Germinal Center)
Activated B Cell
↓
Plasma Cell (Antibody production) OR Memory B Cell (Long-term immunity)