Antibody refers to the protective protein produced by the body due to the stimulation of antigen. It (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 only found in the blood and other body fluids of vertebrates, and the cell membrane surface of B cells. Antibodies can recognize a unique feature of a specific foreign object, and the foreign target is called an antigen.
2. Types of antibodies
Antiboby is a type of immunoglobulin that can specifically bind to antigen.
A. Antibody can be divided into lectin, sedimentation, antitoxin, lysin, opsonin, neutralizing antibody, complement fixing antibody, etc. according to reaction forms.
B. According to the source of antibody, it is divided into normal antibodies (natural antibodies), such as anti-A and anti-B antibodies in blood type ABO, and immune antibodies such as anti-microbial antibodies.
C. According to the source of the reactive antigen, it is divided into heterogeneous antibodies, heterophilic antibodies, homologous antibodies and autoantibodies.
D. According to the agglutination state of the antigen reaction, it is divided into complete antibody IgM and incomplete antibody IgG.
3. The Structure of Antibody
The X-ray crystal diffraction structure analysis found that Ig is composed of four polypeptide chains, which are connected by different numbers of interchain disulfide bonds between the peptide chains. Ig can form a "Y"-shaped structure, called Ig monomer, which is the basic unit that constitutes an antibody.
In addition to the above-mentioned basic structure of Ig light chain and heavy chain, some types of Ig also contain other auxiliary components, such as J chain and secretory tablets. Under certain conditions, certain parts of the peptide chain of Ig molecules are easily hydrolyzed into different fragments by protease. Papain and pepsin are the two most commonly used Ig proteolytic enzymes, which can be used to study the structure and function of Ig to isolate and purify specific 12 polypeptide fragments.
4. The Function of Antibody
The function of an antibody is closely related to its structure. The differences in the amino acid composition and sequence of the V region and the c region of the same antibody determine the functional difference. The V-region and C-region of different antibodies have certain rules in structural changes, which makes them have commonality in function. The composition and structure of the V and C regions determine the biological function of the antibody.
A. Neutralize toxins and prevent pathogen invasion Recognizing and specifically binding to an antigen is the main function of an antibody. The structure that performs this function is the V region of the antibody, in which the CDR part plays a decisive role in the recognition and binding of the specific antigen.
B. Activate complement to produce membrane attack complex to dissolve and destroy cells After IgG1～3 and IgM bind to the corresponding antigen, and the complement binding sites in the CH2 and CH3 domains can be exposed due to conformational changes, thereby activating the complement system through the classical pathway and generating multiple effector functions. IgM, IgG1 and IgG3 have stronger ability to activate the complement system than IgG2.
C. Regulate phagocytosis and ADCC IgG can bind to cells with corresponding receptors on the surface through its Fc segment, producing different biological effects.
a. Opsonization refers to the binding of the Fc segment of IgG antibodies (especially IgG1 and IgG3) to the corresponding Fc receptors on the surface of neutrophils and macrophages, thereby enhancing the phagocytosis of phagocytes.
b. Antibody-dependent cell-mediated cytotoxicity (ADCC) refers to cells with killing activity (such as NK cells) recognize the Fc segment of an antibody coated with a target cell surface antigen, such as a virus-infected cell or tumor cell, and kill the target cells directly through Fc receptors on their surface.
D. Mediates type I hypersensitivity IgE is a cytophilic antibody, which can sensitize it by binding to the IgE high-affinity Fc receptor on the surface of mast cells and basophils through its Fc segment.
E. Cross the placental barrier and mucosa In humans, lgG is the only antibody that can pass through the placenta. The trophoblast cells on the maternal side of the placenta can express a specific IgG transport protein called FcRn. IgG can selectively bind to FcRn, thereby transferring to the trophoblast cells and actively entering the blood circulation of the fetus.
A. Polyclonal antibody Natural antigen molecules often contain a variety of different epitopes. Using this antigen to stimulate the body’s immune system can activate multiple B cell clones at the same time. The antibodies produced will contain multiple antibodies against different epitopes, so it is called polyclonal antibodies. Polyclonal antibodies are mainly obtained from animal immune serum, serum of convalescent patients or immunized population.
B. Monoclonal antibodies Monoclonal antibody technology provides new methods for the diagnosis and treatment of diseases in clinical applications. As a therapeutic drug, monoclonal antibodies are mainly used in the fields of tumors, autoimmune diseases, organ transplant rejection and viral infections. Monoclonal antibodies can also be used for tumor-targeted therapy. It connects a monoclonal antibody against a tumor antigen to chemotherapy or radiotherapy drugs, and uses the specific recognition and binding characteristics of monoclonal antibodies to carry the drug to the target cell and directly kill it.
What is antigen?
1. Antigens Definition
Antigen (abbr. Ag) refers to a substance that can cause antibody production, and is any substance that can induce an immune response. Foreign molecules can be identified by immunoglobulins on B cells or processed by antigen presenting cells and combined with the major histocompatibility complex to form a complex to activate T cells and trigger a continuous immune response.
2. Types of antigens
According to the nature of the antigen, it is divided into two categories: complete antigen and incomplete antigen. Complete antigen, abbreviated as antigen, is a class of substances that are both immunogenic and immunoreactive. For example, most proteins, bacteria, viruses, bacterial exotoxins, etc. are complete antigens. Incomplete antigen, or hapten, is a substance with only immunoreactivity, but no immunogenicity.
According to whether antigens stimulate B cells to produce antibodies require T cells to assist in classification or not, it can be divided into thymus-dependent antigen (TD-Ag) and thymus-independent antigen (TI-Ag). TD-Ag refers to an antigenic substance that requires T cell helper and macrophages to activate B cells to produce antibodies. TI-Ag refers as an antigen that can directly stimulate B cells to produce antibodies without the help of T cells. Features: can only cause humoral immune responses; only produce IgM antibodies; no immune memory.
According to the source of the antigen, the antigen can be divided into: A. Xenoantigens: antigens between different races such as pathogenic microorganisms and toxoids; B. Alloantigens: antigens that exist between different individuals of the same race, such as HLA, ABO blood group antigen, Rh antigen, MHC, etc.; C. Autoantigens: divided into concealed autoantigens, altered autoantigens, etc., such as eye lens protein, etc.; D. Heterophilic antigens: also known as Forssman antigens, which are common antigens with no species specificity on the surface of different species. They can exist in animals, plants, microorganisms and humans, such as hemolytic streptococcus. The common antigen of human endocardium or glomerular basement membrane is heterophilic antigen.
In addition, antigens can be divided into: A. Endogenous antigen: refers to the antigen produced by the target cell of immune effector cell itself; B. Exogenous antigen: refers to an antigen not produced by APC itself. And natural antigen (natural Ag), artificial antigen (artificial Ag), synthetic antigen (synthetic Ag), etc.
3. The Characteristics of antigens
A. Foreign body property refers to the antigenic substance that enters the body tissues, which must be different from the composition of the body tissue cells. Antigens generally refer to foreign substances that enter the body, such as bacteria, viruses, pollen, etc.; antigens can also be substances between different species; substances between allogens can also become antigens; certain isolated components in the body can also become Antigen.
B. Macromolecularity means that the substances constituting the antigen are usually macromolecular substances with a relative molecular mass greater than 10,000. The larger the molecular weight, the stronger the antigenicity. Most proteins are good antigens.
C. Specificity means that an antigen can only specifically bind to the corresponding antibody or effector T cell.
4. The Structure of antigens
Antigens are different from the body itself in chemical structure and have foreign body properties:
A. Foreign substances. From the perspective of biological evolution, the farther the blood relationship between heterogeneous animals, the stronger the immunogenicity. For example, horse serum and various microorganisms are hardly related to humans, so they are highly immunogenic. Horse serum is closely related to donkeys and mules, so the immunogenicity is relatively weak.
B. Allogeneic substances. Such as human red blood antigens material and human leukocyte antigen.
C. Own material. Self-materials are generally not immunogenic. Some substances, such as hidden self-components (eye crystallin, sperm, etc.), are normally isolated from the immune system. But once the barrier is broken, these substances enter the bloodstream and can come into contact with immunocompetent cells and become autoantigens.
Antigen vs Antibody
All substances that can activate and induce an immune response.
A protein that can specifically recognize and bind antigen.
Usually protein, but also can be polysaccharide, lipid, nucleic acid
In vivo or in vitro, mainly in vitro
a. Divided into TD-Ag and TI-Ag according to whether Th cells are required to participate.
b. According to the relationship with the body, it is divided into heterophilic Ag, self-Ag and idiotypic Ag.
c. According to the source of antigen in APC, it is divided into endogenous and exogenous Ag.
IgG, IgA, IgM, IgD, IgE
Specific binding site
Cause illness and allergic reactions
Neutralize antigen, protect antibody
Exist in all types of cells; mainly in viruses, bacteria and fungi.
Only present in certain types of cells.
It is highly variable with different structural conformations and usually consists of different epitopes.
It consists of three main parts:
-Two light chains
-Two heavy chains
Exists due to random mutations in cell genes.
Complex chemicals that bind to very specific antigens.
Usually from foreign objects (viruses, bacteria and mycotoxins).
Naturally produced by the human body (B lymphocytes or B cells).
1. Exogenous antigens: bacteria, viruses, fungi, etc.
2. Endogenous antigens: blood type antigens, HLA (histocompatibility leukocyte antigen), etc.
3. Autoantigens: nucleoprotein, nucleic acid, etc.
Breast milk, tears, saliva, sweat and mucus.
There are many differences between antigen and antibody. Antigen is a stimulant, which can stimulate the body to produce antibodies. While antibody is a kind of effect substance, a kind of protein produced by plasma cells. It mainly exists in body fluids such as serum. It can specifically bind to the corresponding antigen and has immune function.
In normal circumstances, the antigen is a foreign protein. For example, some people are allergic to pollen. Pollen can be seen as an antigen to stimulate the body to produce corresponding antibodies, so there is a fundamental difference antigen vs antibody.