Lymphocytes

  • Introduction to Lymphocytes

    • Lymphocytes play crucial roles in the immune system.
    • The two main types are T cells and B cells, along with natural killer (NK) cells.

  • Morphological Similarities

    • PP cells (plasma cells) and NK cells appear morphologically similar.

  • T Cells

    • T cells mediate immune responses and reduce antibodies.
    • They play a key role in killing tumor cells and pathogen-infected cells.
    • T cells acquire self-identity and recognize self vs. non-self through HLA antigens.
    • Important in organ transplant rejection due to foreign antigens.
    • Key markers include:
    • CD3, CD5, CD7 (pan-T cell markers)
    • CD4 (T helper cells) and CD8 (T suppressor cells)

  • B Cells

    • B cells primarily reside in lymph nodes and do not circulate freely in peripheral blood.
    • When activated, B cells mature into plasma cells located in the bone marrow.
    • Key activation process involves antigen presentation, transforming into either memory cells or antibody-producing plasma cells.
    • Important markers include:
    • CD19 (pan-B cell antigen)
    • CD10 (common acute fibroblastic leukemia antigen, found in precursor B lymphocytes)

  • Leukemia and B Cells

    • Identifying the stage of B cell development using markers (e.g., CD10 expression) helps in diagnosing leukemia.
    • Younger B cells express CD10; older B cells do not.
    • Other markers: CD20, CD21, CD22, and CD24 are associated with mature B lymphocytes.

  • Thymic Function and T Cell Maturation

    • Some precursors migrate from the bone marrow to the thymus, where they proliferate and differentiate into T lymphocytes.
    • Thymus function declines with age but continues to replenish T cells throughout life.

  • T Cell Roles

    • T cells are involved in recognizing foreign antigens, crucial for cell-mediated immunity.
    • In HIV infection, the ratio of CD4 (T-helper) to CD8 (T-suppressor) cells is critical; normal ratio is 2:1. Infected individuals may see this flip.

  • Natural Killer Cells

    • NK cells are part of innate immunity and kill tumor or virus-infected cells without prior sensitization.
    • They arise from the bone marrow and don't need thymic involvement.
    • Markers for NK cells typically require the use of monoclonal antibodies for differentiation.

  • Stages of Lymphocyte Maturation

    • Lymphoblast: Large size, high nuclear-to-cytoplasmic ratio, fine lacy chromatin.
    • Prolymphocyte: Smaller than lymphoblasts, more cytoplasm, nucleoli present, lower NC ratio.
    • Small Lymphocyte: Nucleus occupies most of the cell, chromatin is condensed, little cytoplasm.
    • Reactive Lymphocyte: Increased size in response to infection, cytoplasm may appear basophilic.

  • Significant Terminology

    • Reactive lymphocytes can be either T or B cells.
    • Plasmacytoid lymphocytes are on their way to becoming plasma cells within bone marrow, primarily B cells.
    • Immunoblasts proliferate and mature to either plasma cells or memory cells.

  • Plasma Cells

    • Oval-shaped, off-centered nucleus, prominent Golgi apparatus, functions to produce antibodies.
    • They possess a highly basophilic cytoplasm due to high RNA content.
    • Variations include large granular plasma cells with unique inclusions.

  • Clinical Considerations

    • Reactive lymphocyte presence in peripheral blood suggests viral infections.
    • Important in differential diagnoses when interpreting blood smears during lab work.
    • Understanding specific features and classifications of T and B lymphocytes aids in diagnosing and understanding pathological conditions in patients.