1. Antibody definition

Antibody (immunoglobulin is not just an antibody) is a large Y-shaped protein secreted by plasma cells (effector B cells) and used by the immune system to identify and neutralize foreign substances such as bacteria and viruses. It is found in body fluids such as the blood of vertebrates, and the cell membrane surface of its B cells. Antibodies can recognize a unique feature of a particular foreign object.

Related concepts

Antigen (Ag): Any substance that induces an immune response. The foreign molecule can be identified by immunoglobulin on B cells or treated by antigen-presenting cells and combined with a major histocompatibility complex to form a complex to reactivate T cells, triggering a continuous immune response.

T cells and B cells: immune cells can be roughly divided into two major categories, T cells and B cells, both from the bone marrow, but T cells are formed in the thymus, its main function is to phagocytose foreign invaders. The main function of B cells is to produce a variety of antibodies, just like weapons in the army, so that we can resist foreign invaders. Human B cells can produce more than one billion different types of antibodies. T cells do not produce antibodies but act directly. Therefore, the immune function of T cells is called "cell immunity". B cells act by producing antibodies. Antibodies are present in body fluids, so the immune function of B cells is called "humoral immunity".

2. Structure of the antibody

An antibody is a symmetric structure with four polypeptide chains: two identical heavy chains (H) and two identical light chains (L). The interchains are linked by a disulfide bond and a non-covalent bond to form a monomer molecule composed of four polypeptide chains. The light chain has two kinds of κ and λ, and the heavy chain has five kinds of μ, δ, γ, ε and α.

The entire antibody molecule can be divided into two parts, a constant region and a variable region. In a given species, the constant regions of different antibody molecules have the same or nearly identical amino acid sequences. The variable zone is located at the ends of the arms of the "Y". A small portion of amino acid residues are particularly strongly changed in the variable region, and the residue composition and arrangement order of these amino acids are more likely to occur in the variable region called the hypervariable region. The hypervariable region is located on the surface of the molecule and consists of a range from 17 amino acid residues to 2 or 3 amino acid residues. The hypervariable region amino acid sequence determines the specificity of the antigen. The two antigen-binding sites on one antibody molecule are identical and are located at the ends of the arms and are called antigens.

Related concepts

Fab segment and Fc segment: The Fab segment is a fragment of the antigen binding (Fab), which corresponds to two arms of an antibody molecule, and consists of a complete light and heavy chain VH and CH1 domains. The Fc segment is a fragment crystallizable (Fc) corresponding to the CH2 and CH3 domains of Ig, and is a site where Ig interacts with an effector molecule or a cell. The Fab segment contains the complete variable region and the CH1 region of the constant region. The Fc segment refers only to the region of the Ig constant region CH2 and CH3, which corresponds to the portion below the Y-shaped structure.

3. Antibody classification

（1）By the targeting objects

According to the target, it can be divided into anti-toxin, anti-bacterial antibody, anti-viral antibody and pro-cell antibody (immunoglobulin capable of binding to cells, such as lgE-reactive antibody in type 1 allergy, which can be adsorbed on the target cell membrane)

（2）Animal antibody functions

Antiviral serum antibodies of different immunological properties are obtained depending on the nature of the injected bacteria or virus, and injection into different kinds of animals. Animal serum antibodies on the market are broadly classified into the following types:

1. Porcine antibody: swine fever antibody, swine blue ear antibody, porcine circovirus antibody, pig pseudorabies antibody, porcine parvovirus antibody, swine aftosa antibody, swine flu antibody, etc.

（3）According to physicochemical properties

According to physicochemical properties and biological functions, the antibodies can be divided into IgM, IgG, IgA, IgE, and IGD five kinds.

Antibodies of IgM are the first antibodies secreted in an immune response. They initiate a cascade of complements after binding to the antigen. They also link invaders together and bunch them into a heap that facilitates phagocytosis by macrophages;

The IgG antibody activates complement and neutralizes multiple toxins. IgG lasts for a long time and is the only antibody that protects the fetus from the placenta during pregnancy. They also secrete from the mammary gland into the colostrum to protect the newborn babies.

The IgA antibody enters the mucosal surface of the body, including the mucous membranes of the respiratory, digestive, reproductive, etc., and neutralizes the infectious agents. It can also be delivered to the digestive tract mucosa of newborns through the colostrum of breast milk, which is the most important and most quantity type of antibody in breast milk;

The tail of the IgE antibody binds to the cell membrane of basophils and mast cells. When the antibody binds to the antigen, basophils and mast cells release tissue-like substances to promote the development of inflammation, which is also an antibody that triggers an allergic reaction;

The role of IGD antibodies is not well studied. They mainly appear on the surface of mature B lymphocytes and may be involved in the differentiation of B cells. (IGD was found in human myeloma protein in 1995, and its molecular weight is 175kD. It is mainly produced by atherosclerotic cells such as tonsils and spleen. The concentration of IGD in human serum is 3~40μg/ml, which is less than1% of the total Ig in the serum, synthesized late in the development of the individual. The IgD hinge region is very long and sensitive to protease hydrolysis, so the half-life of IGD is very short, which is only 2.8 days. The exact immune function of IGD in serum is still unclear. In the cell stage, in addition to the expression of SmIgD, the antigen is stimulated to exhibit immune tolerance. SmIgD gradually disappears after activation or activation of mature B cells or into memory cells.

（4）According to the visible reaction

According to whether there is a visible reaction after binding to the antigen, it can be divided into: a complete antibody with visible binding reaction in the presence of medium, that is, the so-called antibody, and Incomplete antibodies, which can suppress the antigen corresponding antibodies without any visible reaction.

（5）By antibody source

According to the source of the antibody, it can be divided into natural antibodies and immune antibodies.

An antibody is an immunoglobulin, the altered globulin molecule. It is produced by specific antigen stimulation. The antibody is produced because the antigen invades the body and causes various immune cells to interact so that the B cells in the lymphocytes differentiate and proliferate to form plasma cells, and the plasma cells can produce secreted antibodies.

（6）Development of the antibody

The development of monoclonal antibodies has gone through four phases: murine monoclonal antibodies, chimeric monoclonal antibodies, humanized monoclonal antibodies, and fully human monoclonal antibodies.

4. Fully human monoclonal antibodies: The variable and constant regions of their antibodies are all human, removing immunogenicity and toxic side effects. Related technologies for the preparation of fully human antibodies include human hybridoma technology, EBV transformed B lymphocyte technology, phage display technology, transgenic mouse antibody production technology (transgenic mouse) and single B cell antibody preparation technology.