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In this article, we will discuss the molecular structure of antibodies and their related functions. In addition to this, we will also outline the hybridoma method for the production of monoclonal antibodies and the principles of using monoclonal antibodies in the diagnosis of disease and the treatment of disease. So, let us get started.
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Structure of Antibodies
- Antibodies refer to the globular glycoproteins known as immunoglobulins which have a quaternary structure
- A quaternary structure is represented as Y-shaped, containing two long peptide chains. These chains are bonded by disulfide bonds to two short polypeptide chains
- Each polypeptide chain contains a constant and a variable region
- The constant regions do not change within a class of antibodies but change between the classes. The mechanism employed to destroy the antigens is determined by the constant region. Remember that there are five categories of mammalian antibodies, and each category (class) has a different role.
- In the variable regions of the antibodies, the amino acid sequence varies for each antibody. It is the variable region where the antibody is attached to the antigen to create an antigen-antibody complex
- A site known as an antigen-binding site is at the end of the variable region. Each antigen binding site is usually made up of 110 to 130 amino acids and encompasses both the ends of the light and heavy chains
- The antigens binding sites differ greatly to give specificity to the antibody so that it can bind to the antigens. The sites are specific to the epitope which is the constituent of the antigen that binds to the antibody
- The hinge region is where the disulfide bonds connect the heavy chains. This region gives flexibility to the antibody molecule which enables the antigen-binding site to be placed at various angles while binding to antigens. This region is not found in all antibody classes.
Functions of Antibodies
- B-lymphocytes produce antibodies
- Antibodies bind to particular antigens that stimulate the specific immune response
- Every antigen has a single antibody
- Pollen, pathogen and their toxins, blood cell surface molecules, and the surface protein present on transplanted tissues are included in the antigens
- Antibodies are classified into five major categories (isotypes). Each category plays a different role.
The functions of antibodies vary:
- Antibodies combine with toxins of pathogens and viruses (for instance, bacteria) to prevent them from entering or damaging the cells.
- Antibodies can play the role of anti-toxins by binding to toxins that are produced by pathogens (For instance, diphtheria and tetanus-causing bacteria) which neutralize them so that they can become harmless
- Antibodies can join bacteria so that they can be recognized by phagocytes. This is known as opsonization. Once recognized, the phagocyte has receptor proteins for the heavy polypeptide chains of antibodies which allows phagocytosis to take place.
- Antibodies can join with the flagella of the bacteria to make them less active. It then makes phagocytosis easier.
- Antibodies play a role in the agglutinins which causes clumping of pathogens carrying antigen-antibody complexes. This is known as agglutination. It minimizes the probability of spreading pathogens through the body and allows phagocytes to engulf several pathogens at one time.
- Antibodies along with other molecules can form holes in the cell walls of the pathogens. The holes can result in the bursting of cell walls when the water is absorbed by osmosis.
Producing Monoclonal Antibodies – The Hybridoma Procedure
- Monoclonal antibodies are produced artificially from one B cell clone
- The hybridoma method is a procedure used to create monoclonal antibodies (Mabs)
- This method allows large quantities of identical antibodies to be created
- The hybridoma method resolved the issue of having B cells that can divide by mitosis but not generate antibodies and plasma cells that can generate antibodies but not divide
- Just like the naturally produced antibodies, the monoclonal antibodies bind antigens
- They are created when mice are injected with an antigen that triggers the production of antibody-producing plasma cells
- Individual (isolated) plasma cells from mice are fused with immortal tumour cells which result in hybridoma cells
- The desired antibody is produced after these hybrid cells are grown in a selective growth medium followed by the screening
- After that, they are cultured to generate a huge number of monoclonal antibodies
- Monoclonal antibodies have several applications which include diagnostics, treatment of a disease, food safety testing, and pregnancy testing
Uses of Monoclonal Antibodies
Diagnosis of Diseases
Monoclonal antibodies can be used for:
- Diagnosing Pregnancy
- Detecting HIV
- Identifying the presence of pathogens like Streptococcus bacteria
- Differentiating between Herpes I and Herpes II
- Tissue typing before transplants
- Blood typing before transfusion
- Identifying the presence of antibodies in milk
- Identifying cancer cells
These antibodies can also be employed to identify the position of the blood clots in patients who may have deep vein thrombosis. It is achieved by:
- Injecting human fibrin in mice (Human fibrin is the primary protein present in blood clots)
- It stimulates plasma cells to generate antibodies against fibrin
- The cells are gathered from the spleen of the mouse
- The plasma cells are then joined with tumour cells to create hybridomas that generate antifibrin antibodies
- To identify the location where the antibodies bind to fibrin molecules, a radioactive chemical is attached to the antibodies. This makes them radioactively labelled.
- A gamma-ray camera is employed to identify where these radioactively labelled antibodies have attached to the fibrin molecule, therefore, pointing toward the location of the blood clots.
- Usually, monoclonal antibodies are used one time only.
Treatment of Diseases
Monoclonal antibodies have several applications therapeutically which include:
- Treating rabies virus by injecting purified antibodies
- Preventing rejection of the transplanted organ. It is accomplished by intervening with the T cells that play a role in the process of rejection
- Autoimmune therapies for rheumatoid arthritis and allergic asthma. Here monoclonal antibodies can bind and deactivate the factors that play their role in the inflammatory response
- Treating diseases that are caused by the overproduction or incorrect production of B cells. These diseases include multiple sclerosis, myasthenia gravis, and leukaemia
- Preventing blood clotting after the angioplasty procedures. It is achieved by binding monoclonal antibodies to the receptors on the platelet surface to prevent fibrinogen from binding and the clotting from ensuing
- Target treatment of breast cancer is achieved by a monoclonal antibody known as Herceptin (trastuzumab). It is an antibody that identifies the receptor proteins on the surface of cancer cells and binds to them, enabling the immune system to recognize and destroy them.
- Treating melanoma which is a type of skin cancer. The antibody which plays the role in the treatment is ipilimumab which binds to a protein produced by T cells. The T-cells minimize the immune response. When this antibody binds to the T-cell, the immune system remains active against the cancer cells.
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