Antibody: Definition, Structure, Diagram, Types, Function

Antibodies, also known as immunoglobulins, are a crucial component of the immune system. They are produced by the body in response to the presence of foreign substances, such as viruses and bacteria, and play a vital role in protecting the body from infection and disease.

In this blog post, we will dive into the world of antibodies, exploring their structure, function, and various types. We will also discuss how they have been produced and the role they play in the immune response.

Additionally, we will explore the use of antibodies in medical research and treatment, such as in the creation of monoclonal antibodies for cancer therapy.

Whether you’re a student studying immunology or simply interested in learning more about the intricacies of the immune system, this post is sure to provide valuable information and insights.

Antibody definition

An antibody, also known as an immunoglobulin, is a protein produced by the body’s immune system in response to the presence of harmful substances, called antigens. Antigens can be a wide variety of substances, such as microorganisms (bacteria, fungi, parasites, and viruses), chemicals, and even certain types of cancer cells.

What is an antibody?

Antibodies are proteins that protect you if an unwanted substance enters your body. Antibodies produced by your immune system bind to these unwanted substances to remove them from your system.

The immune system is responsible for recognizing and responding to these antigens, and antibodies are a key component of this response. When an antigen enters the body, the immune system recognizes it as a threat and begins to produce specific antibodies that can bind to and neutralize the antigen.

what is an antibody

This binding process is critical in the elimination of the antigen from the body. Antibodies can neutralize the antigen by blocking its ability to function or by promoting its removal from the body. Antibodies can also activate other immune cells, such as complement proteins, to help eliminate the antigen.

The antibodies are produced by a certain type of white blood cell called B cells. Each antibody has a unique structure, consisting of two light chains and two heavy chains. At the very tip of the antibody is a hypervariable region, which allows the antibody to make different types of antibodies that will respond to all of the antigens that will assault the body.

An antigen is anything that is foreign to the human body. It can be a virus, it can be a bacteria, and in some cases, the immune system can mistakenly consider healthy tissue in the body as a harmful substance and produce antibodies against it. This is known as an autoimmune disorder and can lead to the development of various diseases such as rheumatoid arthritis, lupus, and multiple sclerosis.

Diagram of antibody

The labeled diagram of an antibody is a visual representation of the structure and components of this important immune system protein. The diagram shows the different regions of the antibody including the variable region, constant region, and the Fc and Fab regions. Additionally, it illustrates the binding sites for antigens.

Labeled diagram of antibody.
what is an antibody?
structure of antibody

Understanding the structure and function of antibodies is essential for understanding how the immune system works and how to effectively design vaccines and treatments for infectious diseases.

Structure of antibody

Each antibody consists of four polypeptides, two heavy chains and two light chains, joined together to form a “Y” shaped molecule. The structure of the antibody is critical for its function as it allows the antibody to bind to a specific antigen. The variable region of the antibody, which is located at the tips of the “Y” shape, is responsible for this specificity.

The variable region of the antibody is composed of 110-130 amino acids, and it varies greatly among different antibodies. This variability in the amino acid sequence is what gives the antibody its specificity for binding to a specific antigen. The variable region includes the ends of the light and heavy chains, and it is this region that is responsible for recognizing and binding to the antigen.

When the antibody binds to an antigen, it can be cleaved by a protease enzyme to produce Fab (fragment antigen binding). This is a fragment of the antibody that still maintains the variable ends of the antibody and its ability to bind to the antigen.

The constant region of an antibody determines the mechanism used to destroy the antigen. The constant region is responsible for the class of the antibody and its immune function. The five major classes of antibodies are IgM, IgG, Iga, IgD, and IgE. Each class of antibody has a unique constant region structure which determines its specific function in the immune response.

The variable region of an antibody is further subdivided into hypervariable (HV) and framework (FR) regions. The hypervariable regions have a high ratio of different amino acids in a given position, relative to the most common amino acid in that position.

This variability in the amino acid sequence is what gives the antibody its specificity for binding to a specific antigen. Within the light and heavy chains of the antibody, three hypervariable regions exist, known as HV1, HV2, and HV3.

The framework regions, on the other hand, have more stable amino acid sequences. The framework regions separate the hypervariable regions, and they are responsible for maintaining the structure of the antibody. They provide a stable framework for the variable regions to bind to the antigen.

The HV regions, also known as the complementarity determining regions (CDRs), are the regions of the variable region that directly contact the surface of the antigen. These regions have a high degree of variability in their amino acid sequence, which allows the antibody to bind to a specific antigen. The HV regions are the most critical part of the variable region for determining the specificity of an antibody.

On the other hand, the FR regions, or framework regions, form a beta-sheet structure which serves as a scaffold to hold the HV regions in position. These regions have a more stable amino acid sequence and are responsible for maintaining the structural integrity of the antibody. The beta-sheet structure formed by the FR regions creates a stable platform for the HV regions to interact with the antigen.

Types of antibodies

IgG, IgM, IgA, IgD, and IgE are the five types of antibodies, also known as immunoglobulins, that are present in the human body. Each of these antibodies has a specific function and is distributed differently throughout the body.

1. IgG.

The main antibody found in the blood, IgG has a powerful ability to bind to bacteria and toxins. It plays an important role in the biological defense system and is the only isotype that can pass through the placenta to protect a newborn.

2. IgM

Constructed of five units of basic Y-shaped structures, IgM is mainly distributed in the blood. Produced first upon pathogen invasion by B cells, it plays a key role in the initial immune system defense.

3. IgA

 Present in blood as monomers and in secretions such as bowel fluid, nasal discharge, and saliva as dimers, IgA is involved in preventing bacterial invasion from a mucous membrane. It is also present in breast milk and protects the gastrointestinal tract of newborns from bacterial and viral infections.

4. IgD

Found on the surface of B cells, IgD is believed to play a role in the induction of antibody production and the prevention of respiratory tract infections.

5. IgE

Originally related to immunity reactions to parasites, IgE is involved in allergies such as pollinosis by binding to mast cells.

Function of antibody

Antibodies, also known as immunoglobulins, are proteins produced by the immune system that protect the body from foreign substances such as pathogens and toxins. They have several different functions, including:

  1. Neutralization. Antibodies are secreted into the blood and mucosa, where they bind to and inactivate foreign substances such as pathogens and toxins, preventing them from causing harm to the body.
  2. Complement activation. Antibodies can activate the complement system, a group of proteins that can directly damage pathogens and infected cells. This can lead to the destruction of bacterial cells by lysis (punching holes in the cell wall).
  3. Phagocytosis. Antibodies can facilitate the ingestion and destruction of foreign substances by phagocytic cells, such as macrophages and neutrophils, by a process called opsonization.
  4. Specificity. Antibodies precisely recognize toxins and pathogens, allowing the immune system to target specific invaders.
  5. Diversity. The immune system can produce a wide variety of antibodies that can target different antigens, ensuring that the body is protected against a range of potential invaders.
  6. Immunological memory. Once an individual has been exposed to a pathogen, their immune system remembers how to recognize and respond to it. This allows the immune system to mount a more efficient response if the individual is exposed to the pathogen again in the future. This is why people who have had measles, for example, do not develop symptoms of the disease again when they are exposed to it later in life.
  7. Immune tolerance. The immune system is able to distinguish between self-cells and tissues and foreign substances, and it does not normally attack the body’s own cells. This is known as immune tolerance and is important for preventing autoimmune diseases.

Antibodies in medicine and research

Antibodies have a wide range of applications in both medicine and research. Preformed antibodies, which are derived from the blood serum of previously infected people or animals, can be administered in an antiserum to provide passive immunization against fast-acting toxins or microbes.

For example, in case of snakebites or tetanus infections, preformed antibodies can be used to provide immediate protection against the toxin or microbe.

Vaccines, on the other hand, are used to confer active immunity against a specific harmful agent by stimulating the immune system to produce its own antibodies against the agent. Once stimulated by a vaccine, antibody-producing B cells remain sensitized and ready to respond to the agent should it ever gain entry to the body.

Monoclonal antibodies, which are artificially produced through genetic engineering and related techniques, are particularly valuable in research and medicine. They have the ability to recognize individual antigenic sites on almost any molecule, which makes them useful in identifying and targeting specific cells, such as cancer cells.

They are produced in large quantities and are highly specific, which makes them useful in sensitive assays for a wide range of biologically important substances.

Monoclonal antibodies have been used experimentally to deliver cytotoxic drugs or radiation to cancer cells. They have also been shown to have great potential in the immune destruction of cancer cells.

Monoclonal Antibodies For Cancer Therapy

Monoclonal antibody therapy is a form of cancer treatment that uses lab-created antibodies to target and destroy cancer cells. These lab-made antibodies are clones or exact copies of a specific antibody that has been designed to bind to and attack specific markers found on the surface of cancer cells.

The process of creating these lab-made antibodies begins with the collection of cells from a patient’s cancerous tumor. These cells are then used to produce large quantities of a specific antibody that is known to target the cancer cells. The resulting antibodies are called monoclonal antibodies, as they are all identical copies of the same antibody.

Once the monoclonal antibodies have been produced, they are purified and tested for safety and efficacy before being used to treat the patient’s cancer. The antibodies can be administered to the patient through an IV infusion, or they can be directly targeted to the cancer cells using a technique called radioimmunotherapy.

Monoclonal antibody therapy can be used to treat many types of cancer, including breast cancer, colorectal cancer, and lung cancer. It has also been used to treat other diseases such as rheumatoid arthritis, multiple sclerosis and more.


American Cancer Society. Monoclonal Antibodies and Their Side Effects. ( Accessed 5/6/2022.

Aziz M, Iheanacho F, Hashmi MF. Physiology, Antibody. ( StatPearls. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Accessed 5/6/2022.

Malik B, Ghatol A. Understanding How Monoclonal Antibodies Work. ( StatPearls. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Accessed 5/6/2022.

Leave a Comment