Lymphocytes and Differential Significance Notes

Lymphocytes & Differential Significance

Learning Outcomes

• Understand the functions of white blood cells and their roles in the immune system.

• Recognize and describe the microscopic appearance of different types of white blood cells.

• Interpret how white blood cells are represented in a full blood count.

• Analyze the diagnostic indications provided by a differential white blood cell count for various diseases.

Cell Lineage

The development of lymphoid cells originates from:

• Multipotent stem cell: These are undifferentiated cells capable of differentiating into various types of blood cells.

• Common lymphoid progenitor (CLP): This progenitor gives rise to:

  • Lymphoblast: A precursor cell that matures into lymphocytes.

  • T lymphocyte (T cells): Key players in cell-mediated immunity, including cytotoxic T cells and helper T cells.

  • B lymphocyte (B cells): Responsible for antibody-mediated immunity.

  • NK cells (Natural Killer cells): Part of the innate immune system, providing cytotoxic responses against infected or cancerous cells.

• Common myeloid progenitor (CMP): Gives rise to myeloid cells, which include granulocytes, monocytes, and macrophages.

Lymphocytes

• Second most numerous WBC in peripheral blood (PB).

• Critical cellular component of the adaptive immune system, which provides specific and long-lasting immunity.

• Variable size, indicating different activation states and subtypes.

• Can be longer-lived than the cells of the innate immune system, providing immunological memory.

• No marginating reserve of mature lymphocytes, meaning they are primarily circulating in the blood and lymphoid organs.

• Types:

  • T Cell: Approximately 75% of lymphocytes, generally smaller than monocytes. Subtypes include helper T cells (CD4+) and cytotoxic T cells (CD8+).

  • B Cell: 10-15% of lymphocytes, also smaller than monocytes. Responsible for producing antibodies and differentiating into plasma cells.

  • Large Granular Lymphocyte (LGL): 10-15%, larger than B/T cells, and primarily composed of NK cells, which have cytotoxic functions.

Lymphocyte Lifespan

• Complex, involving circulation in peripheral blood and residence in lymphoid organs for immunosurveillance.

• Involves various stages and locations:

  • PB (Peripheral Blood): Lymphocytes circulate to monitor for pathogens.

  • Secondary lymphoid organs (lymph nodes/ tonsils/ spleen/ mucosa associated lymphoid tissue/ BM): Sites where lymphocytes encounter antigens and mount immune responses.

  • Different T cell subpopulations (TN, TSCM, TCM, TEM, TEFF) with varying markers (CD45RA, CD95, IL-2RB, CCR7, CD62L):

    • TN (Naive T cells): CD45RA+, CCR7+, CD62L+ – have not yet encountered their target antigen.

    • TSCM (Stem cell memory T cells): CD45RA+, CD95+, IL-2RB+ – capable of self-renewal and differentiation into other T cell subsets.

    • TCM (Central memory T cells): CCR7+, CD62L+ – reside in lymphoid organs and are important for long-term immunity.

    • TEM (Effector memory T cells): CCR7-, CD62L- – migrate to tissues and provide rapid responses to re-infection.

    • TEFF (Effector T cells): Actively involved in eliminating pathogens.

Cell Characteristics and Functions

• T Cells

  • Found in: Peripheral Blood, Thymus.

  • Function: Cytotoxicity (CD8+ T cells) and immune regulation (CD4+ T cells).

    • CD8+ T cells: Directly kill infected cells.

    • CD4+ T cells: Secrete cytokines to activate other immune cells.

• B Cells

  • Found in: Peripheral Blood, Bone Marrow.

  • Function: Antibody Production. Differentiate into plasma cells that produce large quantities of antibodies.

    • Antibodies: Recognize and neutralize pathogens.

• NK Cells

  • Found in: Peripheral Blood.

  • Function: Cytotoxicity. Kill infected or cancerous cells without prior sensitization.

    • Mechanism: Release cytotoxic granules containing perforin and granzymes.

Lymphocyte Count

• High count: lymphocytosis, which can indicate infection, inflammation, or malignancy.

• Low count: lymphopenia, which can result from immunodeficiency, autoimmune diseases, or certain medications.

• Reference ranges:

  • WBC: 4.0 – 11.0 \times 10^9/L

  • Lymphocytes: 1.5 – 4.0 \times 10^9/L

  • Lymphocytes Percentage: 18 – 40%

T cell/B cell morphology

• High nucleus:cytoplasm ratio.

• Condensed chromatin.

• Scanty cytoplasm.

LGL Morphology

• NK cells.

• Round nucleus.

• Abundant cytoplasm.

• Open lacey chromatin.

• May contain cytoplasmic granules.

Lymphocyte Differential

• Flow cytometry for cell markers, used to identify and quantify different lymphocyte subsets.

• Cells are labeled with fluorescent antibodies that bind to specific surface markers.

• Process involves:

  • Cryopreservation: Freezing cells to preserve them for later analysis.

  • Surface Marker Staining: Applying fluorescently labeled antibodies to cell surface markers.

  • Flow cytometer analysis: Passing cells through a laser beam and measuring the fluorescence to identify cell types.

  • Cell Separation: Isolating specific cell populations for further analysis.

  • Intracellular Antigen Staining: Staining for intracellular proteins to further characterize cells.

Cell Markers

• CD3: T Cell (+), B Cell (-), NK (-). Expressed on all T cells and used to identify them.

• CD19: T Cell (-), B Cell (+), NK (-). Expressed on B cells and used to identify them.

• CD56: T Cell (-), B Cell (-), NK (+). Expressed on NK cells and used to identify them.

WBC Differential

• Immunosurveillance: The immune system continuously monitors the body for threats.

• Peripheral blood serves as the transport system for the immune and lymphatic systems, facilitating immune cell circulation.

• Blood sample as a window into the immune system, providing valuable diagnostic information.

Clinical Associations

• Bacterial infection

  • Neutrophilia: Increased neutrophil count.

  • Monocytosis: Increased monocyte count, especially in severe infections.

  • Lymphocytosis: Increased lymphocyte count in some bacterial infections.

• Viral infections

  • Lymphocytosis: Increased lymphocyte count is common.

  • Neutropenia: Decreased neutrophil count in infections like EBV/CMV/hepatitis A & B.

  • Lymphopenia: Decreased lymphocyte count in some viral infections.

• Hypersensitivity reactions

  • Basophilia: Increased basophil count in acute hypersensitivity reactions.

  • Eosinophilia: Increased eosinophil count, commonly seen in skin and lung presentations.

• Helminth infections

  • Eosinophilia: Increased eosinophil count.

  • Basophilia: Increased basophil count.

• Malignant associations

  • Eosinophilia: Associated with Hodgkin lymphoma.

  • Basophilia: Associated with chronic myeloid leukaemia.

  • Neutrophilia: Increased neutrophil count in some malignancies.

• Drug associations

  • Neutropenia: Decreased neutrophil count due to chemotherapy.

  • Monocytopenia: Decreased monocyte count.

  • Lymphopenia: Decreased lymphocyte count.

• Inflammation

  • Neutrophilia: Increased neutrophil count.

  • Lymphocytosis: Increased lymphocyte count.

• Auto-immunity

  • Neutropenia & lymphocytosis: Can occur in autoimmune conditions.