Lymphocytes: T Cells, B Cells, and Natural Killer Cells

Overview of Lymphocytes

  • Lymphocytes are a type of white blood cell crucial for the immune response.
    • Main types: T cells, B cells, Natural Killer (NK) cells.
    • They comprise about 20-30% of circulating white blood cells.

T Cells Overview

  • T Cells are thymus-dependent cells produced in the bone marrow and mature in the thymus.
    • Subsets of T cells include:
    • Cytotoxic T Cells (TC):
      • Function: Attack virus-infected cells through direct contact.
      • Mechanism: Recognize viral antigens presented on the infected cell surface and release cytotoxic chemicals to destroy it.
    • Helper T Cells (TH):
      • Function: Activate B cells and other T cells.
      • Role: Help B cells recognize antigens and initiate antibody production.
    • Suppressor T Cells (TS):
      • Function: Inhibit the immune response from overstimulation.
      • Importance: Prevents potential damage from an overly aggressive immune response.

B Cells Overview

  • B Cells are bone marrow-derived cells that differentiate into plasma cells.
    • Plasma cells secrete antibodies that respond to specific antigens (e.g., from bacteria or viruses).
    • Memory B cells are formed after infection, allowing for rapid antibody production upon subsequent encounters with the same antigen.
    • B cells engulf antigens recognized by their antibodies, thus degrading foreign particles in the body.

Natural Killer Cells Overview

  • Natural Killer (NK) Cells act as the body's surveillance system.
    • Function: Attack foreign cells, virus-infected cells, and cancer cells.
    • Role in cancer: Help identify and destroy emerging cancerous cells in the body.
    • Importance in organ transplantation: Responsible for donor rejection due to recognizing foreign genetic makeup.
    • In autoimmune diseases: Could play a role in damaging the body’s own cells (e.g., diabetes).

Immune Response Mechanisms

T Cell Activation
  • T cells require an antigen-presenting cell (APC), often a dendritic cell, to recognize and respond to pathogens.
    • Steps involve:
    1. Virus infects the APC.
    2. APC processes viral antigens and presents them on its surface.
    3. Activated T cells bind to these antigens, proliferating into:
      • Cytotoxic T cells which destroy infected cells.
      • Memory T cells which persist for long-term immunity.
B Cell Activation
  • B cells respond to antigens found in bodily fluids, unlike T cells that target infected cells directly.
    • Mechanism involves:
    1. Antigen binds to B cell receptors (BCR).
    2. B cells proliferate and differentiate into plasma cells generating antibodies specific to the recognized antigen.
    3. Provides immunity by producing antibodies rapidly upon re-exposure to the same antigen.