Makindo Medical Notes.com

Overview of Lymphocytes

Lymphocytes are a type of white blood cell that play a critical role in the immune system. They are essential for recognizing and responding to pathogens, cancer cells, and other foreign substances.

Types of Lymphocytes

• B Lymphocytes (B Cells):
  • Produce antibodies that bind to specific antigens.
  • Differentiate into plasma cells and memory B cells.
• T Lymphocytes (T Cells):
  • Subdivided into various types with specific functions:
    • Helper T Cells (CD4+): Activate other immune cells by releasing cytokines.
    • Cytotoxic T Cells (CD8+): Destroy virus-infected cells and tumour cells.
    • Regulatory T Cells (Tregs): Maintain immune tolerance and prevent autoimmune diseases.
• Natural Killer (NK) Cells:
  • Innate immune cells that target and destroy infected or cancerous cells without prior sensitization.
  • Recognize stressed cells in the absence of antibodies and MHC.

Functions of Lymphocytes

• B Cells:
  • Antibody Production: B cells differentiate into plasma cells, which produce antibodies that bind to specific antigens, neutralizing pathogens or marking them for destruction by other immune cells.
  • Memory B Cells: Long-lived cells that remember past infections and provide a rapid response upon re-exposure to the same antigen.
• T Cells:
  • Helper T Cells (CD4+):
    • Release cytokines to activate B cells, cytotoxic T cells, macrophages, and other immune cells.
    • Enhance the immune response to infections and tumours.
  • Cytotoxic T Cells (CD8+):
    • Directly kill infected or cancerous cells by inducing apoptosis.
    • Recognize antigens presented by MHC class I molecules on the surface of target cells.
  • Regulatory T Cells (Tregs):
    • Suppress the immune response to maintain tolerance to self-antigens.
    • Prevent autoimmune diseases and limit chronic inflammation.
• Natural Killer (NK) Cells:
  • Recognize and kill virus-infected cells and tumour cells without prior sensitization.
  • Release perforin and granzymes to induce apoptosis in target cells.
  • Produce cytokines such as IFN-γ to enhance the immune response.

Mechanisms of Action

• Antibody-Mediated Immunity (Humoral Immunity):
  • B cells produce antibodies that neutralize pathogens or mark them for destruction.
  • Antibodies can neutralize toxins, block viral entry, and facilitate phagocytosis by opsonization.
• Cell-Mediated Immunity:
  • Helper T cells (CD4+) activate other immune cells through cytokine release.
  • Cytotoxic T cells (CD8+) kill infected or cancerous cells by recognizing antigens presented on MHC class I molecules.
  • Regulatory T cells (Tregs) suppress excessive immune responses to prevent tissue damage and autoimmunity.
• Innate Immune Response:
  • NK cells provide a rapid response to infected or transformed cells without prior exposure.
  • NK cells recognize stressed cells lacking MHC class I molecules and induce apoptosis.

Clinical Relevance

• Immunodeficiency:
  • Primary immunodeficiencies (e.g., Severe Combined Immunodeficiency, SCID) result from genetic defects affecting lymphocyte development or function.
  • Secondary immunodeficiencies (e.g., HIV/AIDS) are acquired conditions that impair lymphocyte function.
• Autoimmune Diseases:
  • Conditions where the immune system mistakenly attacks the body's own tissues (e.g., rheumatoid arthritis, lupus).
  • Involves dysregulation of T cells and B cells leading to tissue damage and chronic inflammation.
• Cancer Immunotherapy:
  • Strategies such as CAR-T cell therapy involve engineering T cells to target and kill cancer cells.
  • Checkpoint inhibitors block proteins that inhibit T cell activation, enhancing the immune response against tumours.
• Vaccination:
  • Vaccines stimulate the production of memory B cells and T cells, providing long-term protection against specific pathogens.
• Allergic Reactions:
  • Hypersensitivity reactions mediated by B cells and T cells responding to harmless antigens (e.g., pollen, food allergens).

Summary

Lymphocytes are critical components of the immune system, encompassing B cells, T cells, and natural killer (NK) cells. They perform essential functions in antibody production, cell-mediated immunity, and innate immune responses. Understanding their roles and mechanisms is vital for diagnosing and treating various immune-related conditions, including immunodeficiencies, autoimmune diseases, cancers, and allergic reactions.