Adaptive immune system B lymphocytes and antibody production Antibodies (also known as immunoglobulins) are produced by a specific cell population, the B lymphocytes, which are differentiated from bone marrow cells. Antibodies recognize a wide range of foreign antigens, including proteins, carbohydrates, lipids, and nucleic acids, and are key factors in the adaptive immune response to foreign antigens. The basic molecular structure of all immunoglobulins contains at least two heavy chains (50,000-70,000 molecular weight each) and two light chains (23,000 molecular weight each), forming a Y-shaped bivalent structure. Part of the region on each chain is highly conserved and is called the constant region (C region). And at the amino terminus of the polypeptide chain (about 100 amino acids), the amino acid arrangement of this region varies, and is called the structure of the variable region antibody. Reprinted from a collection of clinical slides on rheumatic diseases published by the American College of Rheumatology in 1995. Shenzhen Futian People’s Hospital Xiangmi Lake Branch Ye Zhizhong Within the V region, the composition and order of arrangement of amino acid residues in certain regions are more variable than in other regions within the V region, called the highly variable region. the V region is the region where immunoglobulins bind antigens, while the highly variable region of the V region is the region where antigens are specifically recognized. At the hydroxyl terminus of the heavy and light chains are highly conserved constant regions that play a role in other activities of the immune system, such as binding complement C1 fragments, or interacting with immunoglobulin-binding receptors. Immunoglobulin is expressed on the surface of B cells and is acquired by B cells during differentiation and maturation in the bone marrow lumen. The production of intact heavy and light chains is formed by splicing together several small gene fragments to produce a functional gene capable of encoding the generation of intact heavy and light chains containing the V and C regions. The combination of these small gene fragments undergoes a process of chromosome breakage and rejoining, a process known as genetic recombination. The process of genetic recombination of immunoglobulins is precisely regulated to produce different differentiated populations of B lymphocytes (numbering about 106-109) at the time of B lymphocyte differentiation, and the surface of B lymphocytes of different differentiated populations express immunoglobulins containing different antigen-binding sites. Since this recombination process is tightly regulated, only one immunoglobulin receptor is expressed on the surface of each mature B-lymphocyte. Genetic recombination ensures that once a foreign antigen enters, only B cells that can bind this antigen can be activated, and B cells that cannot bind this antigen cannot be activated, a process called antigen-directed clonal selection. There are five main classes of immunoglobulins: IgM, IgG, IgA, IgE, and IgD, each of which has a different structure and function. The main structural feature that distinguishes these five classes of immunoglobulins is the order of arrangement of the C region of the heavy chain. For example, all IgM have a similar heavy chain C region, but differ from the heavy chain C region of other classes of immunoglobulins (IgG, etc.) IgM is a pentameric structure consisting of five individual antibody molecules linked by J-chains. IgM is the first antibody to appear when a foreign pathogen invades and binds to complement to produce an effective effect. IgG is a major component of total antibodies and binds to immunoglobulin receptors on the surface of phagocytes to mediate binding to antigens. IgA is most concentrated in the mucosal lumen and is mainly found in tears, saliva, bronchial fluid, milk, intestinal fluid and other body fluids. IgA is mainly produced by B lymphocytes in the mucosal lumen. IgE is important for the body’s metabolic response and is thought to play a role in the response to antiparasitic infections. IgD is one of the least understood immunoglobulins, it is least abundant in serum, is co-expressed with IgM on the surface of mature B cells, and plays a role in B cell activation.