Immunology: Antibody Structure and Function

Structure and Function

• Antibodies, also known as immunoglobulins (Igs), are Y-shaped proteins produced by plasma cells (a type of B lymphocyte).
• They are crucial components of the adaptive immune system, specifically humoral immunity.
• Antibodies recognize and bind to specific antigens, which are foreign substances such as bacteria, viruses, or toxins.
• This binding initiates a cascade of events that eliminates the antigens.
• The basic antibody structure consists of four polypeptide chains: two identical heavy chains and two identical light chains.
• These chains are linked by disulfide bonds, forming a Y-shaped molecule.
• The variable regions of the light and heavy chains form the antigen-binding site, exhibiting high specificity for a particular antigen.
• The constant regions of the heavy chains determine the antibody's class or isotype (e.g., IgG, IgM, IgA, IgE, IgD).
• Different antibody classes have distinct effector functions, contributing to various aspects of the immune response.

Antibody Classes (Isotypes)

• IgG: The most abundant antibody class, capable of crossing the placenta and providing passive immunity to the fetus.
  • It plays a vital role in opsonization (enhancing phagocytosis) and complement activation.
• IgM: The first antibody produced during an initial immune response.
  • It has a pentameric structure, making it a potent agglutinating agent (clumping together pathogens).
• IgA: Found in mucosal secretions, such as saliva and breast milk.
  • It helps protect mucosal surfaces from pathogens.
• IgE: Involved in allergic reactions and parasitic infections.
  • It binds to mast cells and basophils, triggering the release of histamine and other mediators.
• IgD: Primarily expressed on the surface of B cells, functioning as a B cell receptor.
  • Its role in antibody production and activation is currently less well understood compared to other classes.

Antibody Diversity

• The large repertoire of antibodies is generated through a process called V(D)J recombination, which involves the shuffling of gene segments encoding antibody variable regions.
• This random recombination leads to the production of millions of unique antibody specificities, enabling the immune system to recognize a vast array of antigens.
• Somatic hypermutation further increases antibody diversity and affinity - the strength of the antigen-antibody interaction.
  • This process occurs in germinal centers of lymph nodes, refining antibody responses.

Antibody-Antigen Interactions

• The specificity of antibody-antigen interactions is based on complementary shapes and chemical interactions.
• This binding can lead to different outcomes, depending on the nature of the antigen and the effector functions of the antibody.
• Neutralization: Antibodies bind to viral particles or toxins, blocking their ability to infect cells.
• Opsonization: Antibodies coating pathogens enhance their uptake by phagocytic cells, like macrophages or neutrophils.
• Complement activation: Antibodies bound to antigens activate the complement system, a cascade of proteins leading to pathogen lysis.
• Agglutination: Antibodies bind to multiple antigens, forming complexes that clump together and facilitate the removal of pathogens.
• Antibody-dependent cell-mediated cytotoxicity (ADCC): Antibodies binding to target cells trigger the killing of those cells by cytotoxic lymphocytes (e.g., natural killer cells).

Applications of Antibodies

• Diagnostic tools: Antibodies are used in diagnostic tests for various diseases (e.g., ELISA, immunofluorescence).
• Therapeutic agents: Monoclonal antibodies are used to treat various conditions, such as cancer and autoimmune diseases.
• Research tools: Antibodies are used extensively in research to study immune responses and specific proteins.

This quiz explores the structure and function of antibodies, key components of the immune system. Learn about the Y-shaped proteins produced by plasma cells and their role in recognizing and binding to antigens. Understand the significance of heavy and light chains and the various classes of antibodies in immune response.