6.6

B Cells and Antibody Production

• B cells are crucial for the immune response as they produce antibodies.

• Antibodies help capture and present antigens, activating the nonspecific immune system to combat infections.

Activation of B Cells

• For a B cell to present an antigen, it must interact with a helper T cell that possesses a corresponding T cell receptor (TCR).

• This interaction leads to reciprocal activation:

  • Activation of helper T cell.

  • Release of cytokines from the activated T helper cell, which further activates the B cell.

• B cells present themselves as discovering pathogens, asking helper T cells for further instruction on whether to respond.

Clonal Expansion and Differentiation

• Upon activation, B cells undergo a process known as clonal expansion—producing numerous identical copies (clones) of themselves.

• Activated B cells differentiate into:

  • Memory cells: Store information for faster response upon re-exposure.

  • Plasma cells: Specialized for high production of antibodies.

Functions of Antibodies

• Antibodies serve several functions, including:

  • Blocking the active sites of harmful enzymes.

  • Causing clumping or agglutination of antibody-tagged cells.

  • Activating complement proteins, natural killer cells, and macrophages, enhancing the immune response.

• Antibodies function as opsonins, marking pathogens for destruction.

Antibody Structure

• Antibodies are formed from immunoglobulins and typically have a Y-shaped structure:

  • Composed of two heavy chains (red) and two light chains (blue).

  • Each antibody contains:

    • Constant segment: Common across all antibodies, crucial for activating natural killer cells and complement proteins.

    • Variable segment: Specific to the unique epitope of the antigen, determined through VDJ recombination.

Types of Immunoglobulins

• Different classes of antibodies (immunoglobulins) serve distinct functions:

  • IgM: Pentamer, present in plasma, does not cross membranes.

  • IgG: Monomer, can cross membranes; plays a significant role in general immunity.

  • IgE: Activates basophils, released histamine; plays a role in allergic responses.

  • IgD: Functions as a receptor on B cells to capture antigens.

  • IgA: Secreted into tears and saliva; protects mucosal surfaces in the body.

Class Switching

• Class switching refers to the alteration of an antibody's constant segment while retaining the same variable segment that recognizes a specific antigen.

• This process occurs during the clonal expansion of B cells, allowing for a versatile immune response.

• It is essential to produce antibodies that can act in various compartments throughout the body, such as plasma and secretions.

• Without class switching, the immune response could be limited to a single body compartment, jeopardizing overall defense against infections.