Humoral Response and B Lymphocytes

Overview

• Focus on B lymphocytes (B cells), a subset of white blood cells (lymphocytes).
• B cells originate from bone marrow.
• Named after the bursa of Fabricius.

Structure of B Cells

• B cells have proteins on their surface (~10,000 proteins per cell).
• These proteins are membrane-bound antibodies (immunoglobulins).
• Antibodies are proteins that can also exist freely in the body.

Variable Portions of Antibodies

• Each B cell has one type of membrane-bound antibody.
• Antibodies have a fixed portion (constant) and a variable portion.
• Variable portions differ between B cells, leading to diversity.
• There are approximately 10 billion different combinations of variable portions.
• Variable portions are generated during B cell development through DNA shuffling.

Function of B Cells

• Immune system uses B cells to prepare for various pathogens (e.g., viruses, bacteria).
• B cells bind to pathogens through their variable portions.
• Binding site on pathogen is called an epitope.
• Only the B cell with the matching variable portion will bind to a specific pathogen.

Activation of B Cells

• Binding to a pathogen activates the B cell.
• Activation usually requires helper T cells.
• Activated B cell clones itself and differentiates.
• Differentiation results in two types:
  • Memory cells: Long-lived B cells with the perfect receptor.
  • Effector cells (plasma cells): Antibody factories producing specific antibodies.

Antibody Production

• Effector cells can produce 2,000 antibodies per second.
• Antibodies tag pathogens for destruction and impede their function.
• Antibodies can lead to opsonization (making pathogens easier for phagocytes to consume).
• Antibodies can cause pathogens to clump together, further hindering their ability to infect cells.

Importance of Diversity

• The diversity of B cell receptors allows the immune system to recognize a wide range of pathogens.
• Mechanism ensures that self-reactive B cells are weeded out during development.
• This system is crucial for the humoral immune response but does not address intracellular pathogens or cancer cells.

Future Topics

• More details on the role of helper T cells in B cell activation.
• Mechanisms to deal with intracellular pathogens and cancer cells.