IMSE Lecture Week 03: B Cell Maturation

Innate Immune System

• Functions from birth, providing a rapid, non-specific response to pathogens.
• Divided into external and internal systems.

External Innate Immune System

• Acts as the first line of defense against pathogens.
• Key components include:
  • Skin
  • Mucous membranes
  • Normal flora
  • Secretions

Internal Innate Immune System

• Utilizes phagocytic cells to engulf and digest invading pathogens.
• Antigen-Presenting Cells (APCs) carry antigen information to the adaptive immune system for antibody production.

Adaptive Immune System

• Targets specific pathogens and creates memory for quicker future responses.
• T cells release cytokines to activate B cells after interacting with APCs.
• B cells produce antibodies for rapid response to re-infection.
• Antibodies enhance the function of the internal innate immune system.

Key Players in Innate Immunity

• Natural Killer (NK) cells attack virus-infected and cancerous cells.
• Pathogen-associated Molecular Patterns (PAMPs) can detect various microbial structures (e.g., peptidoglycan, lipoproteins).
• Toll-like receptors (TLRs) recognize pathogenic products, enhancing immune response.

Organs of the Immune System

• Divided into primary and secondary lymphoid organs for maturation and activation of B and T cells.

Primary Lymphoid Organs

• Bone Marrow:
  • Largest tissue, producing blood cells.
  • Comprises yellow (fat) and red (blood) marrow; yellow marrow increases with age.
• Thymus:
  • Small, bilobed organ in the thorax, shrinks with age and functions also as an endocrine organ.

Secondary Lymphoid Organs

• Spleen:

  • Largest secondary lymphoid organ; filters blood, removing old or damaged cells.
  • Contains red pulp (for culling) and white pulp (lymphoid tissue).
  • T and B cells are located strategically around arterioles and in follicles.

• Lymph Nodes:

  • Junctional filters of the lymphoid system.
  • Ideal for antigen contact; filters lymph fluid.
  • Composed of outer cortex with macrophages, T cells in paracortex, and germinal centers with B cells.

• MALT (Mucosal-associated lymphoid tissue):

  • Located in gastrointestinal, respiratory, and urogenital tracts.
  • Contains structures like tonsils, appendix, and Peyer’s patches.

B Cell Maturation

• Pro-B Cell:

  • Influenced by growth factors like IL7.
  • Markers include CD19, CD45R, and RAG enzymes.
  • Main role is gene rearrangement for antibody heavy and light chains.

• Pre-B Cell:

  • Characterized by a receptor made of heavy and surrogate light chains.
  • Main function is the synthesis of the heavy chain (μ).

• Immature B Cell:

  • Contains complete IgM, CD21, and Class II MHC.
  • Prepares for antigen-specific responses.

• Mature B Cell:

  • Expresses complete IgM and IgD with MHC Class II.
  • Waits in secondary lymphoid organs for activation.

• Plasma Cell:

  • Produces abundant antibodies but has little surface Ig.
  • Found in germinal centers or bone marrow; crucial for antibody production.

Antibody Structure

• Comprised of a tetrapeptide with heavy and light chains.
• Heavy chains consist of five antigenic types (Ig gamma, alpha, mu, delta, epsilon).
• Light chains are classified into two types: kappa and lambda.