Immune Activities of B Cells

Chapter 15 - Immune Activities of B Cells

15.4 Immune Activities of B Cells

  • The section outlines the functions and activities of B cells within the immune system, delving into their development, activation, and how they contribute to immunity.

B Cells

  • Development:
    • B cells develop in the bone marrow.
  • Function:
    • They act as antigen-presenting cells.
    • B cells bind to specific antigens using their B cell receptors (BCRs), which are also known as Immunoglobulins (Igs).

B-Cell Activation

  • Initial Recognition:

    • B cells recognize and bind to microbes and their antigens using their immunoglobulin receptors (Ig).
    • The process involves the endocytosis of the microbe, where the B cell internalizes it, processes the antigen, and displays it on the MHC-II complex.

  • T Helper Cell Interaction:

    • For full activation, B cells must interact with a T helper cell that possesses receptors for the same antigen.
    • The binding occurs through the antigen:MHC II complex that connects with the T helper cell antigen and CD4+ receptors.

Effects of B Cell Activation

  • Once activated, B cell activation leads to:
    • Cell enlargement: Cells increase in size.
    • Proliferation: The proliferation of activated B cells occurs through numerous mitotic divisions, resulting in a greater population of cells carrying the same specificity for the antigen.
    • Differentiation: The outcome of this process results in the creation of memory B cells and plasma cells.

B Cell Activation Creates Memory B Cells and Plasma Cells

  • Clonal Expansion:
    • Activated B cells undergo numerous mitotic divisions, which expand the clone of cells with the same specificity and produce memory cells that can respond to future encounters with the same antigen.
  • Memory B Cells:
    • These are persistent cells that provide long-term immunity by being ready to respond upon re-exposure to the same antigen.
  • Plasma Cells:
    • Plasma cells are short-lived, active cells that immediately secrete antibodies.
    • They produce antibodies (e.g., IgM) that share the same specificity as the original B cell receptor and circulate through bodily fluids to interact with specific antigens.
    • One plasma cell can generate 2,000 antibodies per second.

Characteristics of Plasma Cells

  • Plasma cells are:
    • Large, specialized, and terminally differentiated B cells.
    • Secrete a high volume of antibodies specific to the original receptor of the B cell.
    • They are designed for rapid antibody production and secretion, and they deteriorate shortly after their function.

Characteristics of Memory Cells

  • Memory cells are:
    • Not fully differentiated, but can live for long periods, remaining ready to react to the same antigen later.
    • Upon re-encounter with the antigen, they will expand and create additional cells specific to that antigen.

Overview of B Cell Activation and Antibody Production

  • The process progresses as follows:
    • B cells process the antigen (Ag) shown.
    • B cells interact with T helper (TH) cells.
    • B cells undergo stimulation by growth and differentiation factors.
    • Resulting in cell cycle entry for clonal expansion, leading to the production of both plasma cells (which secrete antibodies) and memory cells (which react to the same antigen in the future).

Antibody Structure and Functions

  • Understanding the structure and function of antibodies is critical to comprehending their role in immune responses.

Antibody-Antigen Interactions

  • Opsonization:
    • When bacteria are opsonized with antibodies, they are engulfed more readily by phagocytes.
  • Neutralization:
    • Antibodies prevent viruses from attaching to their target cells by filling surface receptors or blocking active sites.
  • Complement Fixation:
    • Antibodies bound to a microorganism activate the complement system, leading to cell lysis of certain pathogens (e.g., bacteria and viruses).
  • Agglutination:
    • Antibodies aggregate and link cells or particles, forming clumps that immobilize microbes and facilitate phagocytosis.
  • Precipitation:
    • Involves antigens that are smaller than a cell (typically proteins) being bound by antibodies.
  • Antitoxin:
    • A specific type of precipitation where antibodies can bind bacterial exotoxins or animal toxins, blocking them from causing harm to host cells.

Goals of Antibodies

  • The primary goal of secreted antibodies is to effectively bind to the antigen that prompted their production.
    • Complement Fixation: Helps in the lysis of infected cells.
    • Opsonization: Increases the efficiency of phagocytosis of microbes.
    • Neutralization: Prevents pathogens from infecting cells.

Additional Functions

  • Agglutination: Creates immobilization of pathogens.
  • Precipitation: Promotes phagocytosis of smaller antigens.
  • Antitoxin Activity: Neutralizes harmful toxins before they cause cellular damage.

Classes of Immunoglobulins (Ig)

  • IgG:
    • Monomeric; produced by plasma cells during the primary immune response and memory cells during the secondary response; most prevalent immunoglobulin.
  • IgA:
    • Monomer in blood circulation; dimerized in mucous and serous secretions, important for mucosal immunity.
  • IgM:
    • Pentamer; produced first after encountering an antigen, useful in the initial immune response.
  • IgD:
    • Monomer; acts primarily as a receptor for antigens on B cells.
  • IgE:
    • Monomer; involved in mediating allergic responses and defense against parasitic infections.
    • The primary distinctions between these classes arise from variations in their Fc (fragment crystallizable) regions.

Summary of Immunoglobulin Classes Characteristics

  • A table may be presented later summarizing the various immunoglobulins, comparing their functions and characteristics to give a clearer overview.