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Antibody: the story on HearLore | HearLore
Antibody
In 1890, two scientists named Emil von Behring and Kitasato Shibasaburō discovered that a substance in blood could neutralize deadly toxins from diphtheria and tetanus, launching the field of immunology. This substance, later named the antibody, was not a cell but a protein floating freely in the blood, a concept that challenged the prevailing belief that immunity was solely the job of living cells. The term antibody itself was coined by Paul Ehrlich in 1891, derived from the German word Antikörper, though the word was initially met with skepticism and replaced by various other terms like Immunkörper and Amboceptor before settling into its current form. The discovery revealed that the immune system possessed a sophisticated chemical language, capable of recognizing and neutralizing foreign invaders with a precision that would later be compared to a lock and key mechanism. This initial finding set the stage for a century of research that would reveal antibodies as the primary defenders of the humoral immune system, existing in the body's fluids to identify and neutralize antigens ranging from bacteria to viruses.
The Y-Shaped Architect
An antibody is a heavy protein, approximately 150 kilodaltons in weight and about 10 nanometers in size, arranged in a distinctive Y shape that serves as the blueprint for its function. This structure consists of four polypeptide chains: two identical heavy chains and two identical light chains, all connected by disulfide bonds. The tips of the Y, known as the Fab regions, contain variable domains that form the antigen-binding site, while the trunk of the Y, called the Fc region, determines how the immune system responds to the bound target. The variable domains contain three hypervariable regions, or complementarity-determining regions, which loop out to create a specific shape that complements the antigen's epitope. This structural diversity allows the human body to generate about 10 billion different antibodies, each capable of binding a distinct epitope, despite the limited number of genes available in the human genome. The hinge region between the Fab and Fc parts provides flexibility, allowing antibodies to bind to pairs of epitopes at various distances and form complexes that can agglutinate pathogens or trigger immune cell activity.
The Genetic Lottery
The ability to generate billions of unique antibodies from a relatively small number of genes relies on a complex genetic process known as V(D)J recombination. In the bone marrow, each developing B cell randomly selects and combines one variable, one diversity, and one joining gene segment to create a unique immunoglobulin variable region. This process, which involves cutting and rejoining DNA strands with the help of RAG proteins, ensures that each B cell produces antibodies containing only one kind of variable chain, a phenomenon known as allelic exclusion. Once a B cell is activated by an antigen, it undergoes somatic hypermutation, a process where the genes encoding the variable domains undergo a high rate of point mutation, introducing slight amino acid differences in the antibody chains. This mutation process creates a diverse pool of antibodies, some with weaker interactions and others with stronger binding affinities. B cells expressing high affinity antibodies receive strong survival signals, while those with low affinity die by apoptosis, a process called affinity maturation that allows the average affinity of antibodies to increase over time.
Who discovered antibodies and when was the discovery made?
Emil von Behring and Kitasato Shibasaburō discovered antibodies in 1890. This discovery revealed that a substance in blood could neutralize deadly toxins from diphtheria and tetanus.
What is the structure and weight of an antibody?
An antibody is a heavy protein approximately 150 kilodaltons in weight and about 10 nanometers in size. It is arranged in a distinctive Y shape consisting of four polypeptide chains connected by disulfide bonds.
How does the body generate billions of unique antibodies from limited genes?
The body uses a complex genetic process known as V(D)J recombination to create unique immunoglobulin variable regions. This process involves cutting and rejoining DNA strands with the help of RAG proteins to ensure each B cell produces antibodies containing only one kind of variable chain.
What are the five classes of antibodies and their specific functions?
Antibodies are categorized into five classes: IgM, IgG, IgA, IgE, and IgD. IgM is the first antibody produced during an immune response, IgG is the most abundant class, IgA is found in mucosal areas, IgE is responsible for allergic responses, and IgD functions mainly as an antigen receptor on B cells.
How long do memory B cells and long-lived plasma cells persist in the body?
Long-lived plasma cells can live for potentially the entire lifetime of the organism and reside in survival niches within the bone marrow or mucosal tissues. Memory B cells can be rapidly recalled in a secondary immune response to produce antibodies with higher affinity.
What medical applications do antibodies have for disease diagnosis and treatment?
Antibodies are used to diagnose infections like Epstein-Barr virus and Lyme disease and to screen for blood transfusion compatibility. Monoclonal antibodies are employed to treat diseases such as rheumatoid arthritis, multiple sclerosis, psoriasis, and various forms of cancer.
Antibodies are categorized into five distinct classes or isotypes, each with unique biological properties, functional locations, and abilities to deal with different antigens. IgM is the first antibody produced during an immune response, existing as a pentamer with five Ig units capable of binding up to 10 epitopes, making it highly effective at eliminating pathogens in the early stages of infection. IgG is the most abundant class, providing the majority of antibody-based immunity and being the only antibody capable of crossing the placenta to give passive immunity to the fetus. IgA is found in mucosal areas such as the gut, respiratory tract, and urogenital tract, preventing colonization by pathogens and also found in saliva, tears, and breast milk. IgE is the least abundant class, responsible for allergic responses and the release of histamine from mast cells, though it also plays a role in protecting against parasitic worms. IgD functions mainly as an antigen receptor on B cells that have not been exposed to antigens, activating basophils and mast cells to produce antimicrobial factors. Each class is defined by the constant region of the heavy chain, which determines the effector function triggered when the antibody binds to an antigen.
The Memory of the Body
The immune system's ability to remember past infections is largely due to the existence of memory B cells and long-lived plasma cells that can persist for decades. When a B cell is activated, it can differentiate into a plasma cell that secretes huge quantities of antibody regardless of whether antigen is present, ensuring that antibody levels do not fall to zero. Long-lived plasma cells can live for potentially the entire lifetime of the organism, residing in survival niches within the bone marrow or mucosal tissues. Memory B cells, on the other hand, can be rapidly recalled in a secondary immune response, undergoing class switching and affinity maturation to produce antibodies with higher affinity. This memory mechanism is central to the efficacy of vaccines, which aim to elicit persistent high levels of antibody production. However, many microbes can mutate to escape antibodies elicited by prior infections, and long-lived plasma cells cannot undergo affinity maturation or class switching, a limitation compensated for by the adaptability of memory B cells.
The Battle Within
Antibodies function through a variety of mechanisms to neutralize and eliminate pathogens, including neutralization, agglutination, precipitation, and complement activation. Neutralizing antibodies block parts of the surface of a bacterial cell or virion to render its attack ineffective, while agglutination causes antibodies to glue together foreign cells into clumps that are attractive targets for phagocytosis. Precipitation forces serum-soluble antigens to clump together and precipitate out of solution, and complement activation encourages the complement system to attack the pathogen with a membrane attack complex. Antibodies can also trigger antibody-dependent cell-mediated cytotoxicity, where natural killer cells release cytokines and cytotoxic molecules to destroy invading microbes. The Fc region of the antibody binds to Fc receptors on effector cells such as macrophages, neutrophils, and natural killer cells, triggering specific immune responses. This interaction allows the immune system to invoke only the appropriate mechanisms for distinct pathogens, ensuring a targeted and effective defense against a wide range of threats.
The Medical Revolution
The understanding of antibodies has led to groundbreaking medical applications, from disease diagnosis to targeted therapies. Detection of particular antibodies is a common form of medical diagnostics, used to diagnose infections like Epstein-Barr virus and Lyme disease, and to screen for blood transfusion compatibility. Monoclonal antibodies, which are identical antibodies produced by a single B cell, are employed to treat diseases such as rheumatoid arthritis, multiple sclerosis, psoriasis, and various forms of cancer. The development of Rho(D) immune globulin antibodies has prevented hemolytic disease of the newborn by destroying Rh antigens in the mother's system before they can stimulate the immune system to attack future pregnancies. Research antibodies are used to identify and locate proteins within cells, differentiate cell types, and separate proteins from cell lysates. The ability to engineer antibodies, such as creating heterodimeric or bispecific formats, has opened new avenues for cancer treatment and drug delivery, allowing for the attachment of different combinations of drugs to the arms of the antibody.