11.05 B-Lymphocytes (B cells)

Types of Lymphocytes

Overview:

• Lymphocytes are a type of white blood cell smaller than phagocytes.
• Characterized by a large nucleus relative to their cell size.

Main Types of Lymphocytes

1. B-Lymphocytes (B Cells)
   • Maturation: Occur in the bone marrow.
   • Primary Locations: Concentrate in lymph nodes and the spleen.
   • Function: Each B cell is specific to one antigen and produces a corresponding antibody.
2. T-Lymphocytes (T Cells)
   • Maturation: Occur in the thymus gland.
     • Thymus Growth: Grows until puberty, then gradually shrinks.
   • Subtypes & Functions:
     • Helper T Cells: Coordinate immune responses by stimulating B cells.
     • Cytotoxic T Cells: Directly destroy infected or cancerous cells.

B-Lymphocyte Maturation & Function

Diversity of B Cells

• Variety: Approximately 10 million different B cell types.
• Specificity: Each B cell has a receptor specific to one unique antigen.

Clonal Selection and Expansion

1. Clonal Selection:
   • B cells with receptors that match an encountered antigen are selected for activation.
2. Clonal Expansion:
   • Selected B cells undergo rapid mitosis to increase their numbers, ensuring a sufficient immune response.

B Cell Receptors (BCRs)

• Specificity: Each BCR is tailored to bind a single specific antigen.

Key Processes in B Cell Function

1. Primary Immune Response
   • Initial Encounter: The first exposure to a new antigen.
   • Characteristics: Involves slow clonal expansion and formation of plasma and memory cells.
2. Secondary Immune Response
   • Subsequent Encounters: Upon re-exposure to the same antigen.
   • Characteristics: Faster and more robust response due to the presence of memory cells.

B Cells in the Immune Response

1. Antigen Recognition
   • Specific B cells bind to their matching antigen via BCRs.
2. Clonal Selection and Mitosis
   • Selected B cells undergo mitotic division to produce:
     • Plasma Cells: Short-lived cells that produce large quantities of antibodies.
     • Memory Cells: Long-lived cells that provide immunological memory for rapid response upon re-exposure.
3. Antibody Secretion
   • Plasma Cells:
     • Production Rate: Capable of producing thousands of antibodies per second.
     • Distribution: Antibodies are secreted into the bloodstream, lymphatic system, and mucosal linings.
   • Memory Cells:
     • Longevity: Can persist in the body for years or decades.
     • Function: Enable a swift and effective secondary immune response upon re-infection with the same antigen.

Adaptations and Mechanisms

• Plasma Cells:
  • Adaptations: Possess extensive endoplasmic reticulum and ribosomes to facilitate high-rate antibody production.
• Memory Cells:
  • Role in Immunity: Serve as the foundation for long-term immunity, ensuring the body can quickly respond to previously encountered pathogens.

Key Terms

• Pathogen: An organism that causes disease.
• Antigen: A molecule on the surface of a pathogen that triggers an immune response.
• Clonal Selection: The process by which lymphocytes with receptors specific to an antigen are selected for activation.
• Clonal Expansion: The rapid division of selected lymphocytes to increase their numbers.
• Primary Immune Response: The initial response to a new antigen, characterized by slower activation and antibody production.
• Secondary Immune Response: A faster and more potent response upon re-exposure to the same antigen, mediated by memory cells.
• Plasma Cell: A differentiated B cell that produces and secretes antibodies.
• Memory Cell: A long-lived B cell that remains in the body to provide rapid responses upon future exposures to the same antigen.

Immune Response Summary

Steps in B Cell Activation:

1. Antigen Binding:
   • The B cell receptor binds to its specific antigen.
2. Mitosis:
   • The activated B cell undergoes mitotic division to produce plasma and memory cells.
3. Antibody Secretion:
   • Plasma Cells: Begin producing and secreting specific antibodies to neutralize the antigen.
4. Memory Activation:
   • Memory Cells: Remain in the body to enable a quick and effective response if the same antigen is encountered again.

Examples of Immune Memory

• Measles:
  • Single Strain: Immune memory effectively prevents re-infection after recovery.
• Colds & Flu:
  • Multiple Strains: Due to various strains with different antigens, each strain may require a primary response, leading to recurrent illnesses.

Questions Analysis

• Mitosis in Immune Response:
• Purpose: Ensures a rapid increase in the number of specific B cells to effectively target and eliminate the antigen.
• Plasma Cell Adaptations:
• Features: Extensive endoplasmic reticulum and abundant ribosomes allow plasma cells to produce antibodies efficiently and in large quantities.