Structure and Functions of Immunoglobulin
Immunoglobulins, also known as antibodies, are Y-shaped proteins produced by plasma cells that are key players in the body's immune response against antigens. The basic unit of an antibody is composed of four peptide chains - two identical heavy chains and two identical light chains connected by disulfide bonds. The N-terminal ends of both the heavy and light chains have a variable region that provides the ability to bind to different antigens. The remaining parts of the heavy and light chains make up the constant region which determines the class or isotype of the antibody such as IgG, IgM, IgA, IgD, and IgE.
Based on Immunoglobulin molecular structure, antibodies carry out important functions within the immune system. By recognizing and binding specifically to antigens through their variable regions, antibodies help eliminate pathogens through various mechanisms. For example, binding of antibodies to bacteria or viruses can activate the complement system, leading to lysis of the pathogen's cell membrane. Antibody binding can also mediate phagocytosis as they mark antigens for uptake and destruction by macrophages or neutrophils. Antibodies neutralize toxins and viruses by interfering with their ability to bind host cells. The Fc regions of some antibody classes also activate effector cells like natural killer cells to directly lyse target cells.
Generation of Antibody Diversity
To defend against the vast array of pathogens the body may encounter, the immune system requires a diverse repertoire of antibodies capable of recognizing nearly any foreign antigen. This diversity is generated through genetic events occurring in developing B cells within the bone marrow and lymphoid tissues. During early B cell development, V(D)J recombination randomly rearranges variable (V), diversity (D), and joining (J) gene segments to generate a vast number of variable gene combinations for the heavy and light chains. Subsequent somatic hypermutation further diversifies these genes in germinal centers during T cell-dependent immune responses, introducing point mutations in the variable regions. Together, V(D)J recombination and somatic hypermutation allow a single B cell to produce antibodies with affinities broad enough to bind virtually any antigen.
Antibody Isotypes of Immunoglobulin
Beyond differences in their variable regions, antibodies also have unique functions depending on their isotype or class. The five major classes found in humans are:
- IgG: The most abundant isotype in serum and extracellular fluid. IgG antibodies play a key role in secondary immune responses by activating the complement cascade and opsonizing pathogens.
- IgM: The first isotype to appear during primary responses. As a pentamer, IgM is highly efficient at cross-linking antigens but its large size prevents deep tissue penetration. It is important for clearing pathogens in early infection.
- IgA: Found predominantly in secretions such as saliva, tears, and breast milk. IgA antibodies provide mucosal immunity against invading pathogens in the respiratory, intestinal, and urogenital tracts.
- IgD: Found on the surface of naïve B cells where it may play a role in B cell activation and differentiation. IgD levels drop sharply after primary antigen exposure.
- IgE: Mediates allergic hypersensitivities and defenses against parasites. Its high-affinity receptors on mast cells and basophils trigger release of inflammatory mediators during allergic reactions.
Measuring and Diagnosing with Antibodies
Techniques like enzyme-linked immunosorbent assay (ELISA) and Western blotting allow quantification of antibodies specific to a target antigen in body fluids like serum, saliva, or cerebral spinal fluid. Measurement of antibody levels plays an important role in many applications:
- Diagnosis of infections: Serology rapidly detects antibodies produced against bacterial or viral infections to confirm exposure history.
- Diagnosis of autoimmune diseases: Autoantibodies indicate underlying conditions like systemic lupus erythematosus or rheumatoid arthritis.
- Monitoring transplants: Detection of donor-specific antibodies can predict rejection risks in organ transplant recipients.
- Assessment of vaccine responses: Antibody titers after vaccination determine degree and durability of immune protection conferred.
- Identification of allergens: Specific IgE antibodies point to allergens causing hypersensitivity reactions.
Antibodies form the cornerstone of adaptive immunity through their exquisite antigen specificity and diversity of functions. Detection and measurement of antibody isotypes by serology provides invaluable diagnostic information across various clinical contexts. The immunoglobulin would be severely compromised without these versatile Y-shaped molecules defending us against pathogens.
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