What is Antigen and Antibodies | Immune System

What is Antigen and Antibodies

The immune system is our body’s natural defense against harmful invaders such as bacteria, viruses, fungi, and toxins. Two of its most critical components are antigens and antibodies. While the terms are often mentioned together, they play very different but complementary roles in immunity.

What is an Antigen?

An antigen is any molecule or molecular structure that triggers an immune response in the body. Usually, antigens are found on the surface of pathogens (microbes causing disease) or are toxins secreted by them. When the immune system detects an antigen, it treats it as “foreign” and works to neutralize or destroy it.

Structure of Antigens

An antigen’s structure determines how the immune system recognizes it and how effective it is at triggering a response. While antigens can vary greatly depending on their origin (bacteria, viruses, pollen, toxins, etc.), they share some common structural principles.

1. Chemical Nature

• Proteins: Most potent antigens because of their complex three-dimensional shapes and high molecular weight.
• Polysaccharides: Found on bacterial cell walls; often combined with proteins to increase antigenicity.
• Lipids & Nucleic Acids: Usually weak antigens unless linked to proteins or polysaccharides (forming glycolipids, lipoproteins, or nucleoproteins).

2. Molecular Size and Complexity

Effective antigens are typically large molecules (over 10,000 Daltons). Greater structural complexity means more potential binding sites for immune recognition.

3. Epitopes (Antigenic Determinants)

These are specific regions on the antigen’s surface that are recognized and bound by antibodies or T-cell receptors. One antigen can have multiple epitopes, allowing it to interact with several different antibodies.

Conformational Epitopes is formed by the 3D folding of the antigen, while linear Epitopes is formed by the sequence of amino acids in a protein chain.

4. Hapten-Carriers

Haptens are small molecules that are not antigenic by themselves but become antigens when bound to larger carrier molecules (often proteins). 

For example, Penicillin can act as a hapten, triggering an allergic response when bound to body proteins.

5. Structural Location

The antigens are of two types based on their location:

1. Surface Antigens are located on the membrane of pathogens (e.g., viral spike proteins, bacterial capsule polysaccharides).
2. Secreted Antigens are toxins or enzymes released by microbes that can act as antigens.

Functions of Antigens

Antigens play a central role in the immune system, acting as triggers and targets in immune responses. Their functions include:

1. Immune System Activation

Antigens signal the immune system that a foreign invader is present. They initiate both innate (non-specific) and adaptive (specific) immune responses.

2. Specificity in Immune Recognition

Each antigen has unique epitopes, which ensures that immune responses are highly targeted. This specificity allows the immune system to distinguish between harmful pathogens and the body’s own cells.

3. Antibody Production Trigger

Antigens stimulate B lymphocytes to produce antibodies that match their specific epitopes. This process forms the basis of immunological memory—the ability to respond faster upon repeated exposure.

4. Cellular Immune Response Activation

Some antigens are processed and presented on major histocompatibility complex (MHC) molecules to T cells, activating cytotoxic or helper T-cell responses.

5. Role in Vaccination

Vaccines contain harmless forms of antigens (weakened, killed, or fragmented pathogens) that train the immune system to recognize and fight the real pathogen later.

6. Disease Diagnosis

Detecting specific antigens in blood, urine, or tissues helps identify infections (e.g., hepatitis B surface antigen test).

Types of Antigens

Antigens can be classified in several ways based on their origin, location, and the immune response they provoke.

1. Based on Origin

a. Exogenous Antigens

• Enter the body from outside.
• Include bacteria, viruses, fungi, allergens, and toxins.
• Recognized by immune cells after being processed and presented on MHC (Major Histocompatibility Complex) molecules.

b. Endogenous Antigens

• Produced inside body cells due to infection or abnormal processes.
• Examples: Viral proteins synthesized inside infected cells, abnormal tumor cell proteins.
• Presented on MHC class I molecules to cytotoxic T cells.

c. Autoantigens

• Normal self-proteins mistakenly targeted by the immune system.
• Cause autoimmune diseases like lupus and rheumatoid arthritis.

d. Tumor Antigens

• Found on cancer cells.
• Can be either tumor-specific antigens that are present only on tumor cells or tumor-associated antigens that are also found on some normal cells but in abnormal quantities.

2. Based on Immune Response

a. T-dependent Antigens

• Require help from T helper cells for B cells to produce antibodies.
• Usually proteins with complex structures.

b. T-independent Antigens

• Can directly activate B cells without T cell assistance.
• Usually polysaccharides or lipopolysaccharides.

3. Based on Molecular Nature

a. Complete Antigens

• Can provoke an immune response on their own.

b. Incomplete Antigens (Haptens)

• Small molecules that are not immunogenic alone but become antigens when bound to carrier proteins (e.g., penicillin allergy).

What is an Antibody?

An antibody (also called an immunoglobulin) is a Y-shaped protein produced by certain immune cells called B lymphocytes. Antibodies are designed to recognize specific antigens and bind to them to neutralize or mark them for destruction by other immune cells.

Structure of Antibodies

Antibodies, also known as immunoglobulins (Ig), are specialized proteins produced by B lymphocytes (specifically plasma cells) in response to antigens. Their structure is highly specialized for recognizing and binding to foreign molecules with extreme precision.

1. Basic Shape

Antibodies have a Y-shaped structure. This structure allows them to bind antigens at the tips of the “Y” and interact with immune system components at the stem.

2. Polypeptide Chains

Each antibody is made up of four polypeptide chains:

• Two Heavy Chains (H):  Larger chains (~50 kDa each) forming the inner core of the “Y”.
• Two Light Chains (L): Smaller chains (~25 kDa each) attached to each heavy chain.

The heavy and light chains are linked by disulfide bonds for stability.

3. Regions of the Antibody

a.Variable Region (V region)

• Located at the tips of the “Y” arms.
• Highly diverse in amino acid sequence, giving each antibody a unique antigen-binding site.
• Contains the paratope — the part that binds directly to the antigen’s epitope.

b. Constant Region (C region)

The rest of the heavy and light chains have a constant amino acid sequence within each class of antibody. Responsible for determining the antibody’s class (IgG, IgA, IgM, IgE, IgD) and function. The stem of the “Y” is part of the constant region and interacts with immune system components.

c. Hinge Region

They contain flexible segment between the Fab (antigen-binding) and Fc (crystallizable) regions. It allows the arms of the antibody to move and bind antigens that are spaced differently.

4. Antibody Fragments

When antibodies are enzymatically cleaved, they produce:

• Fab Fragment: The “arms” that bind antigens.
• Fc Fragment: The “stem” that interacts with immune cells and complement proteins.

5. Classes of Antibodies

• IgG: Most abundant; long-term immunity; crosses the placenta.
• IgA: Found in mucous membranes, saliva, tears, and breast milk.
• IgM: First antibody produced during infection; good at clumping pathogens.
• IgE: Involved in allergic reactions and defense against parasites.
• IgD: Functions mainly as a receptor on immature B cells.

Functions of Antibodies

Antibodies are critical weapons in the immune system’s arsenal. Their main functions include:

1. Neutralization

Antibodies bind directly to toxins or pathogens, blocking their ability to enter or damage cells. For example, antibodies against influenza virus prevent it from attaching to respiratory cells.

2. Opsonization

Antibodies coat pathogens, making them easier for phagocytes (like macrophages) to engulf and destroy.

The Fc region binds to receptors on immune cells, enhancing pathogen uptake.

3. Agglutination

Antibodies bind to multiple pathogens at once, causing them to clump together. This makes it easier for immune cells to capture and remove them.

4. Complement System Activation

The Fc region can activate a cascade of complement proteins, leading to the destruction of the pathogen’s membrane.

5. Antibody-Dependent Cellular Cytotoxicity (ADCC)

Antibodies mark infected or abnormal cells for destruction by natural killer (NK) cells.

6. Immunological Memory

Once antibodies are produced for a specific antigen, memory B cells can rapidly produce the same antibodies upon re-exposure.

This principle is the basis of long-lasting immunity from infections and vaccines.

Types of Antibodies (Immunoglobulins)

Antibodies are divided into five main classes based on differences in their heavy chain structure and immune functions.

1. IgG

• Structure: Monomer.
• Location: Found in blood, extracellular fluid, and lymph.
• Functions:
• Most abundant antibody (about 70–80% of total antibodies in blood).
• Provides long-term immunity after infection or vaccination.
• Crosses the placenta to provide passive immunity to newborns.
• Activates complement and aids in opsonization.

2. IgA

• Structure: Usually a dimer in secretions; monomer in blood.
• Location: Found in mucosal surfaces, saliva, tears, breast milk, and respiratory/gut secretions.
• Functions:
• First line of defense at mucosal surfaces.
• Prevents pathogens from attaching to epithelial cells.
• Protects infants via breastfeeding.

3. IgM

• Structure: Pentamer (five antibody units joined together).
• Location: Circulates in the blood and lymph.
• Functions:
• First antibody produced during an initial infection.
• Very effective at agglutination (clumping pathogens).
• Strong activator of the complement system.

4. IgE

• Structure: Monomer.
• Location: Found in blood in very small amounts; binds to receptors on mast cells and basophils.
• Functions:
• Involved in allergic reactions (binds allergens and triggers histamine release).
• Provides defense against parasitic worms.

5. IgD

• Structure: Monomer.
• Location: Found in small amounts in blood; mostly on the surface of immature B cells.
• Functions:
• Acts as a receptor for antigens on B cells.
• Plays a role in initiating B cell activation.
• Importance of Antigens and Antibodies

Importance of Antigens and Antibodies

Antigens and antibodies are two of the most critical elements in the immune system’s ability to defend the body. Their interaction forms the foundation of immunity, disease prevention, and many diagnostic and therapeutic tools in modern medicine.

1. Defense Against Pathogens

Antigens

• Act as markers that signal the presence of harmful invaders like bacteria, viruses, fungi, or parasites.
• Each antigen is unique to a pathogen, allowing the immune system to identify the exact threat.

Antibodies

• Recognize and bind specifically to antigens, neutralizing them directly or marking them for destruction.
• Prevent pathogens from attaching to and infecting host cells.
• Example: Antibodies against measles virus prevent the virus from binding to respiratory tract cells, stopping infection before it begins.

2. Basis of Vaccination

Vaccines contain weakened, killed, or fragment forms of pathogens that act as antigens without causing disease.

These antigens stimulate the immune system to produce specific antibodies and memory cells.

This provides long-term protection, as the body can mount a rapid response upon re-exposure to the real pathogen.

Example: The COVID-19 vaccine introduces the spike protein antigen, prompting antibody production.

3. Medical Diagnostics

Detecting specific antigens in a sample can confirm the presence of an infection. For example, detection of Hepatitis B surface antigen confirms active hepatitis B infection.

Detecting specific antibodies indicates past or current exposure to a pathogen. For example, HIV antibody tests determine if someone has been infected with HIV.

4. Monitoring and Managing Diseases

Antibody levels can help monitor immune status after vaccination or infection. While antigen detection can track disease progression or treatment effectiveness. 

For example, tumor antigen levels (like PSA for prostate cancer) help monitor cancer treatment success.

5. Role in Autoimmune Disorders

When the immune system mistakenly identifies the body’s own components as antigens, it produces autoantibodies.

This leads to autoimmune diseases like lupus, rheumatoid arthritis, or type 1 diabetes. Understanding these faulty antigen-antibody interactions helps in diagnosis and treatment planning.

6. Allergy and Hypersensitivity Reactions

Allergens (special types of antigens) trigger the production of IgE antibodies. IgE binds to mast cells, releasing histamine and causing allergic symptoms. Knowledge of the specific allergen–antibody relationship helps in allergy testing and immunotherapy.

7. Therapeutic Uses

Monoclonal antibodies (lab-made antibodies) target specific antigens for treatment. They are used in cancer therapy, autoimmune diseases, and infections.

Antibodies can be engineered to deliver drugs directly to diseased cells by binding to target antigens. For example, Rituximab is a monoclonal antibody that binds to CD20 antigen on certain cancer cells.

8. Forensic and Research Applications

Antigen-antibody interactions are used in forensic science for blood typing and detecting specific substances.

In research, antibodies are tools for detecting, isolating, or measuring specific proteins.

Conclusion

Antigens act as the “ID cards” of foreign invaders, telling the immune system what to fight. Antibodies are the precision weapons, designed to recognize those ID cards and neutralize the threat. 

Together, they make the immune system both highly specific and extremely effective, which is why they are central to immunology, medicine, and biotechnology.

Short Questions and Answers

1. What is an antigen?

A. An antigen is a foreign substance, such as a protein or polysaccharide, that triggers an immune response. It is usually found on the surface of pathogens or as toxins they release.

2. What is the main function of antibodies?

A. Antibodies specifically bind to antigens to block their harmful effects. They also help mark pathogens for destruction by immune cells.

3. Name the most abundant antibody in the blood.

A. IgG is the most common antibody, making up about 70–80% of total antibodies in the bloodstream. It provides long-term immunity after infection or vaccination.

4. What part of the antigen is recognized by an antibody?

A. Antibodies recognize and bind to specific regions on an antigen called epitopes. These are unique molecular structures that determine immune specificity.

5. Which antibody is involved in allergic reactions?

A. IgE antibodies bind to allergens and trigger the release of histamine from mast cells. This causes allergy symptoms like itching, swelling, and sneezing.