Immunoglobulin Ig is composed of four polypeptide chains connected by interchain and intrachain disulfide bonds:

Two identical peptide chains with relatively high molecular weight (referred to as heavy chain, H chain, with a relative molecular weight of approximately 55000 or 70000) and two identical peptide chains with relatively low molecular weight (referred to as light chain, L chain, with a relative molecular weight of approximately 24000).

Let's imagine antibodies as thousands of double toothed forks floating in human blood. Each tooth of each fork can bond to a specific part of a specific antigen, allowing a double toothed fork to bind to two identical antigens. Antigens can be foreign or pathological proteins, and even include RNA, DNA, or polysaccharides from invading pathogens such as fungi, bacteria, viruses, etc.

        Using a double toothed fork as an analogy is just a simplified model. In fact, antibody molecules are very elastic and flexible, and their shape is not a clear "Y" shae. Under an electron microscope, it looks more like the character "pin" formed by stacking three balls.

Antibodies are relatively large molecules with a molecular weight of around 150KD, which is over 8000 times that of water molecules. It is about 4 nanometers wide and about 11 nanometers long. If we imagine a platelet cell as the size of a watermelon, then an antibody molecule is only the size of a watermelon seed.    

1. Types of antibodies

The type of antibody heavy chain directly determines the type of antibody. Mammalian antibody heavy chains can be divided into five categories, represented by the Greek letters γ, α, μ, δ, and ε, respectively. Based on this, antibodies are correspondingly classified into IgG, IgA, IgM, IgD, and IgE. Alpha and gamma contain approximately 450 amino acids, while μ and ε contain approximately 550 amino acids; At the same time, the μ chain and ε chain contain 5 peptide rings, while the γ chain, α chain, and δ chain contain 4 peptide rings.

--IgG is a major Ig in serum, accounting for approximately 65-75% of the total Ig. Widely distributed in tissue fluid, with roughly equal distribution in the intravascular and extracellular spaces. It is an important substance for the body to resist infections.

--IgM is the first type of Ig synthesized by mature fetuses, and it is also the earliest Ig produced after infection or immunity. Among the five types of Ig, IgM is the strongest, so its cytotoxic activity and cell lytic activity are also the strongest. The natural blood type antibody is IgM, and some autoantibodies such as antiphospholipid antibodies, RF, etc. also belong to IgM. Elevated IgM antibodies in fetal umbilical cord blood are a sign of fetal infection.

--The content of IgA in serum and tissue fluid is relatively low, with serotype IgA accounting for 15-25% of total Ig. However, its content is higher in exocrine fluids such as colostrum, saliva, tears, intestinal secretions, and bronchial secretions. Due to the fact that IgA mainly exists in exocrine fluid, it plays an important role in first-line anti infection defense.

--IgE is a monomeric structure and is the least abundant Ig in normal human serum. Its content is extremely low in serum and tissue fluid, and its main biological function is to bind to specific receptors on the surface of mast cells and eosinophils in tissues. IgE cannot activate complement. The IgE content fluctuates greatly in the normal population and can increase in the serum of patients with specific allergic reactions and early parasitic infections. When allergens react with IgE bound to receptors, they can cause degranulation of mast cells and eosinophils, releasing bioactive substances such as histamine and serotonin.

--The concentration of IgD in normal human serum is very low and almost undetectable. IgD mainly exists on the surface of human B lymphocytes as a cell receptor for antigens, with extremely low levels of IgD in serum and different structures from membrane-bound IgD. The variable region of IgD on B cells is the same as the variable regions of IgG, IgA, and IgM that the cell will secrete. When the antigen binds to the IgD receptor, it stimulates B cell proliferation, differentiation, and secretion of other types of antibodies specific to the antigen.

Mammalian antibody light chains consist of approximately 211-217 amino acid residues, with two types: kappa (κ) and lambda (λ). The type of light chain on the same natural Ig molecule is always the same. But there may be antibody molecules with separate kappa or lambda chains in the same body. The ratio of two types of light chains in different species of organisms varies. The ratio of normal human serum immunoglobulin kappa chain to lambda chain is about 2:1, while in mice it is 20:1. Iota (I) subtype light chains exist in other lower vertebrates, such as cartilaginous fish and bony fish.

2. Structure of antibodies

2.1 Variable Region (V Region)

The main structural difference between antibody forks is the variable region located at the tooth tip, which determines the antigen specificity of the antibody. The H chain variable region contains approximately 118 amino acid residues, while the L chain variable region contains approximately 108-111 amino acid residues. The variable region is the location where an antibody binds to an antigen, and its amino acid composition and arrangement determine the antigen binding specificity of the antibody.

There are regions in the variable region where certain amino acids can undergo high-frequency changes and combinations, known as hypervariable regions (HVR). HVR, also known as complementarity determining regions (CDRs), determines the unique type of antibody (anti idiotypic antibody expression). The amino acid composition and arrangement of non HVR regions in the variable region are relatively conserved, known as the framework region (FR region).

There are three CDRs in VL, usually located at amino acid residues 24-34 and 89-97, respectively. Among them, CDR3 has a higher degree of hypermutation, and the maturation of antibody specificity and affinity mainly involves the modification of this region.

2.2 Constant Region (C Region)

The stem of the antibody fork is composed of a series of regions with essentially unchanged amino acid sequences - we call them constant regions. The constant region can interact with cell surface receptors or complement system proteins, triggering host effector functions such as lysis of invading cells or phagocytosis of foreign pathogens.

The antibody constant region is located at 3/4 or 4/5 of the H chain near the C-terminus (approximately from amino acid position 119 to the C-terminus) and 1/2 of the L chain near the C-terminus (containing approximately 105 amino acid residues). The antibody heavy chain is constantly distinguished into CH1, CH2, and CH3, where the CH3 region is involved in receptor binding on the cell membrane surface, and CH2 is involved in the complement activation pathway and is a complement binding site. Simply put, the fork teeth are responsible for recognizing antigens, while the fork handle helps the host decide how to handle antigens.

2.3 Antibody Fab and Fc Segments

IgG is cleaved by papain into two identical Fab fragments and one Fc fragment, each of which is monovalent and can bind to the antigen without further agglutination reaction; After digestion by pepsin, it is cleaved into a complete F (ab) 2 fragment and fragmented Fc fragment. The F (ab ') 2 fragment is bivalent and can simultaneously bind to two antigenic epitopes.

The Fab segment is a fragment of antigen binding (Fab), equivalent to the two arms of an antibody molecule, consisting of a complete VH and CH1 domain of light and heavy chains. The Fc segment is a fragment crystallizable (Fc) segment that corresponds to the CH2 and CH3 domains of Ig, and is the site where Ig interacts with effector molecules or cells. The Fab segment contains the complete variable region and the CH1 region of the constant region. The Fc segment only refers to the region of the Ig constant region CH2 and CH3, which is equivalent to the part below the Y-shaped structure.

2.4 Structure of Heavy Chain Antibodies

The naturally missing light chain antibodies in camels and cartilaginous fish, also known as heavy chain antibodies, are found in camel families such as Camelus bactrianus in Asia, C. dromedarius in Africa, and Lama glama, L. guanicoe, Vicugna pacos, and V. vicugna in South America.

In 1995, new or nurse shark antigen receptors (NAR) similar to heavy chain antibodies without light chain or other protein molecules were discovered in cartilaginous fish such as nurse sharks (Ginglymostoma cirratum), spotted sharks (Oretolobus maculatus), and silver sharks. Due to the similarity between NAR molecules and Ig subtypes in several functional characteristics such as transmembrane and secretory pathways, it is also known as immunoglobulin new antigen receptor (IgNAR).

The single domain heavy chain antibody (sdAb) part has wide application value. Single domain heavy chain antibody refers to a genetically engineered antibody composed only of the variable region of the heavy chain antibody, also known as VHH antibody (variable domain of heavy chain of heavy chain antibody, VHH antibody) or nanobody (Nb), with a molecular weight of 12-15 kD.

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