Lecture Notes: Components of the Adaptive Immune Response

Overview

• Adaptive Immune System: Comprises mainly three types of cells:
  1. B lymphocytes (B cells): Responsible for humoral immunity.
  2. T lymphocytes (T cells): Constitute the cellular arm of adaptive immunity, do not produce antibodies.
  3. Antigen Presenting Cells (APCs): Crucial for T cell activation, do not respond directly to antigens.

Lymphocytes: B and T Cells

• Common Development Pattern:
  • Origin: Both originate in red bone marrow.
  • Maturation:
    • B cells mature in bone marrow.
    • T cells mature in the thymus.
    • Maturation takes 2-3 days.
  • Purpose of Maturation:
    • Immunocompetence: Ability to recognize and bind to a specific antigen.
    • Self-tolerance: Unresponsiveness to self-antigens to prevent autoimmunity.

Steps in Lymphocyte Life Cycle

1. Seeding Secondary Lymphoid Organs and Circulation

   • Naive lymphocytes circulate to increase chances of encountering antigens.
   • Secondary organs include lymph nodes and spleen.

2. Antigen Encounter and Activation

   • Occurs in lymph nodes, spleen, or secondary lymphoid organs.
   • Clonal Selection: Antigen binding selects lymphocyte for proliferation.

3. Proliferation and Differentiation

   • Activated lymphocytes divide into clones with identical antigen specificity.
   • Clones become either effector cells or memory cells.

Antigen Receptor Diversity

• Determined by genes, not antigens.
• Somatic Recombination: Shuffling of genes to create diverse receptors.

Lymphocyte Education

• T cell Education: Best understood in T cells; similar in B cells.
  • Positive Selection: T cells recognizing self-MHC proteins survive.
  • Negative Selection: T cells binding to self-antigens are eliminated (apoptosis).
  • Only 2% of T cells become immunocompetent and self-tolerant.

Types of APCs

1. Dendritic Cells

   • Found in connective tissues, skin.
   • Phagocytic, link innate and adaptive immunity.

2. Macrophages

   • Activate naive T cells and enhance phagocytic abilities.
   • Trigger inflammatory response.

3. B Lymphocytes

   • Present antigens to assist their own activation.

Comparison of B and T Lymphocytes

• B Lymphocytes:

  • Humoral immune response.
  • Target extracellular pathogens.
  • Effector cell: Plasma cell.

• T Lymphocytes:

  • Cellular immune response.
  • Target intracellular pathogens (e.g., viruses).
  • Effector cells: Cytotoxic T cells, Helper T cells, Regulatory T cells.

Summary

• Both B and T cells originate in red bone marrow.
• Differentiation occurs in different locations (bone marrow for B cells, thymus for T cells).
• Efficient antigen presentation and receptor diversity are key to adaptive immunity function.