Antigen - Antibody Reactions - Overview & Types

The immune system is the defence mechanism of the body that protects against pathogens. A crucial part of this defense involves the interaction between antigens and antibodies. This specific binding between antigens and antibodies is known as the antigen-antibody reaction, which plays a key role in identifying and eliminating harmful agents. This Ag-Ab reaction is one of the most significant reactions in immunology.

This Story also Contains

1. What is Antigen-Antibody Reaction?
2. Types of Antigens in Antigen-Antibody Reactions
3. Mechanism of Antigen-Antibody Reactions
4. Antigen-Antibody Reaction Applications
5. MCQs on Antigen-Antibody Reaction
6. Recommended Video on Antigen-Antibody Reactions

The antigen-antibody reaction is vital for studying immunity and disease prevention. It forms the basis of vaccination and is crucial in identifying diseases like AIDS and HIV, where the immune response is compromised. It helps to know about how this reaction works, how the body detects infections, remembers past pathogens, and protects against future attacks.

What is Antigen-Antibody Reaction?

An antigen-antibody reaction is specific and involves the binding of antibodies with antigens. This reaction plays a vital role in providing an immune response to remove foreign substances from the body.

Antigen-Antibody Reaction Definition: A reaction between antigen and antibody, whereby the latter takes hold of the antigens, neutralizes or destroys them, and prevents infections and diseases.

Antigen and Antibody Interaction: The combination between antigen and antibody is very specific. Antibodies bind only those antigens that match their binding sites.

Types of Antigens in Antigen-Antibody Reactions

Antigens are substances that trigger an immune response in the body. They can be classified based on their source or how the body reacts to them. Some antigens come from outside the body, while others originate within, or some are mistakenly recognized as foreign. Different antigens can bring about an immune response and hence produce different types of antigen-antibody reaction.

Exogenous Antigens: The external antigens, like bacteria, and viruses, as well as an allergen, that enter the body.

Endogenous antigens: These are endogenous antigens, naturally occurring within the body due to infected cells or cells which are cancerous.

Autoantigens: These are body proteins recognized by the immune system as foreign, leading to autoimmune diseases.

Alloantigens: These are antigens that arise from the same species but genetically different individuals, such as those used in organ transplants.

Types of Antigen-Antibody Reactions

Antigen-antibody reactions can be of various types based on the nature of the interaction between antibodies and antigens. Each type helps the body eliminate antigens in different ways. Thesereactions are also widely used in medical diagnostics and immunological tests. There are mainly the following types:

Precipitation Reactions: This is the type that occurs when soluble antigens react with antibodies, resulting in an insoluble complex that precipitates out of the solution.

Agglutination Reactions: Antigen-antibody interactions cause clumping of cells or particles, which can be exploited in blood typing and other applications to detect pathogens.

Neutralization Reactions: An antibody will neutralize the pathogenic effects of or the toxin of the pathogen by binding to these. In doing so, it prevents the interaction with host cells.

Opsonization Reactions: Antibodies bind to antigens for better phagocytic recognition and ingestion.

Complement Fixation Reactions: The antigen-antibody complexes activate the complement system, leading to the lysing of the pathogens.

Mechanism of Antigen-Antibody Reactions

This reaction involves the binding of a specific antibody to an antigen. The immune system identifies and responds to the antigen through this interaction. It leads to neutralization, destruction, or removal of the antigen. This mechanism forms the basis of many diagnostic tests and immune responses. The mechanism includes following steps:

Recognition: An antibody correctly identifies a given antigen due to the exact fit between the antigen and its antibody binding sites.

Binding: Once identified, the antibody goes ahead to bind with the antigen in the formation of an antigen-antibody complex.

Immune Response Activation: It may activate various immune responses, including complement activation, phagocytosis, and neutralization, as the complex forms.

Elimination: Neutralization, destruction, or elimination of the bound antigen from the body through effector cells.

Antigen-Antibody Reaction Applications

Antigen-antibody reactions have wide applications in medical and research fields. They are used in blood typing, disease diagnosis, and detecting infections. These reactions also play a vital role in vaccination and allergy testing. Some of the applications include:

Diagnostic Tests: It is used in tests like ELISA and Western Blotting for the detection of various acute and chronic diseases, infections, and allergens.

Vaccine Development: Antigen-antibody interactions help in the development of vaccines that stimulate the production of specific antibodies.

Blood Typing: Agglutination reactions are used to determine blood groups, thus compatibility before transfusion.

Monoclonal Antibody Production: Antibody-antigen reactions are used for the production of monoclonal antibodies for the treatment of diseases such as cancer and autoimmune disorders.

Amensalism in Antigen-Antibody Reactions

Amensalism refers to the interaction in which one organism is harmed or destroyed and the other is not affected. It can also be associated with immune responses. For example:
Immune Response Suppression: Even in viral diseases, pathogens often produce substances that suppress the immune response without doing anything to help the pathogen itself.
Antibody Production Suppression: Some pathogens can suppress antibody production, thus dimming the effect of an immune response.

MCQs on Antigen-Antibody Reaction

Q1. Assertion: Antigens are molecules that can stimulate an immune response in the body.

Reasoning: Antigens are recognized by the immune system as foreign, and can stimulate the production of antibodies or activate immune cells to attack the antigen.

1. Both Assertion (A) and Reason (R) are true and Reason (R) is a correct explanation of Assertion (A).

2. Both Assertion (A) and Reason (R) are true but Reason (R) is not a correct explanation of Assertion (A).

3. Assertion (A) is true and Reason (R) is false.

4. Assertion (A) is false and Reason (R) is true.

Correct answer: 1) Both Assertion (A) and Reason (R) are true and Reason (R) is a correct explanation of Assertion (A)

Explanation:

Antigens are molecules that are recognized by the immune system as foreign or non-self. These molecules can come from a wide range of sources, including pathogens such as bacteria and viruses, as well as cancer cells and even transplanted tissues. When an antigen enters the body, it can be recognized by specialized cells of the immune system, called antigen-presenting cells (APCs), which display the antigen on their surface to alert other immune cells.

In response to the antigen, the immune system can produce antibodies, which are specialized proteins that can recognize and bind to the antigen. Antibodies can neutralize the antigen by preventing it from infecting cells or can mark it for destruction by other immune cells. In addition to antibodies, antigens can also activate immune cells such as T cells and B cells, which can directly attack the antigen and the cells that harbor it.

It's important to note that not all molecules can act as antigens. To stimulate an immune response, an antigen must be recognized as foreign by the immune system, which requires the presence of certain structures or features that are not present in the body's molecules. These structures, called epitopes, can be recognized by antibodies and immune cells, and are often found on the surface of pathogens.

Hence, the correct answer is option 1) Both Assertion (A) and Reason (R) are true and Reason (R) is a correct explanation of Assertion (A).

Q2. The antigen-binding site of an antibody is present at

1. The constant region

2. The C-terminal

3. The variable region

4. Between constant and variable region

Correct answer: 3) The variable region

Explanation:

• The portion of the antibody that is subject to change and is in charge of identifying and attaching to particular antigens—such as germs—is known as the variable region.

• All antibodies have the same constant area, which aids in antibody recognition by the body's immune cells.

• The antibody's C-terminal is merely its last segment and is not capable of binding to antigens.

• Although it aids in structure, the area between the constant and variable sections does not bind to antigens directly.

Hence, the correct answer is option 3) The variable region.

Q3. Antibody is a ___ molecule.

1. Lipid

2. Protein

3. Nucleic Acid

4. Carbohydrate

Correct answer: 2) Protein

Explanation:

Antibody (Ab) is also known as an immunoglobulin(Ig). These are large, Y-shaped blood proteins produced by plasma cells. They bind to foreign particles and invade them. These particles are foreign bodies that get attacked by Antibodies.

Antigens are foreign pathogens that invade the body and have the capability to give rise to a response from our immune system either by grouping up with a larger molecule or alone after binding with antibodies for a particular immune response. Hence, antigens stimulate the production of antibodies by the immune system.

Hence, the correct answer is option 2) Protein.

Also Read:

• MCQs on Antigens

• Bacterial Diseases in Humans

• Plasmodium Life Cycle

Recommended Video on Antigen-Antibody Reactions