Antigen-Antibody Reactions. Principle and Applications of Agglutination. Precipitation and Flocculation Reactions. IMMUNOLOGY AND MYCOLOGY IVth Semester UNIT IV.

Antigen-Antibody Reactions

Antigen-antibody reaction is also known as immune reaction or antigen-antibody interaction. It plays a crucial role in the immune system to recognize and destroy foreign substances in the human body.

Antigen: An antigen is a foreign substance that stimulates an immune response inside the human body. It is a molecule or molecular structure found on the surface of pathogens such as bacteria, viruses, fungi, or other foreign entities. Antigens are also present on the surface of non-pathogenic substances, like pollen, certain foods, or even cells and tissues from another organism.

Antibody (Immunoglobulin): Antibodies are proteins produced by the immune system in response to the presence of antigens. They are also known as immunoglobulins (Ig). Antibodies are highly specific to the antigens they recognize. Each antibody is designed to bind to a particular antigen.

Types of Antibodies: There are several types of antibodies such as IgG, IgM, IgA, IgD, and IgE. Each has distinct roles in the immune response.

Antigen-Antibody Binding: The binding of an antibody to its specific antigen is a highly specific process. The shape of the antigen-binding site on the antibody is matching to the shape of the antigen. This binding is reversible, meaning that the antibody can attach to and detach from the antigen.

Functions of Antigen-Antibody Reactions:

• 1. Neutralization: Antibodies neutralize the harmful effects of toxins produced by pathogens or viruses.

• 2. Agglutination: Antibodies cause pathogens to clump together. This makes it easier for the immune system to eliminate them.

• 3. Opsonization: Antibodies tag pathogens for destruction by phagocytes (A White blood cell that engulfs and digests foreign particles is called phagocytosis).

• Memory Response: Once the immune system responds to a specific antigen, it retains a "memory" of it. If the same antigen reappears, the immune system recognizes the antigen and destroys it.

Principle and applications of agglutination

A process to Clump or aggregate formation of particulate antigens is called agglutination in antigen-antibody reaction. The principle behind agglutination in antigen-antibody reactions is the specific binding of antibodies to antigens. This leads to the formation of visible clumps. This reaction is commonly used in diagnostic tests and laboratory assays. The following are the steps of the agglutination reaction

• 1. Introduction of Antigen: Entry of particulate antigens, such as cells or particles coated with specific antigens.

• 2. Addition of Antibodies: Antibodies specific to the antigen combine with antibodies. These antibodies may be naturally produced by the immune system or provided as part of a diagnostic reagent.

• 3. Agglutination: Antibodies and antigens bind together to form visible clumps or aggregates.

• 4. Visualization: The clumps are visually observed either macroscopically or microscopically.

Applications of Agglutination in Antigen-Antibody Reactions:

• 1. Blood Typing: Agglutination reactions are widely used in blood typing. Different blood groups are determined by the presence or absence of specific antigens on red blood cells. Agglutination tests, such as the ABO and Rh typing are used to identify blood groups for blood transfusion.

• 2. Immunohematology: Agglutination reactions are essential in crossmatching for blood transfusions. Compatibility between donor and recipient blood is determined by observing agglutination reactions to avoid transfusion reactions.

• 3. Microbiology and Serology: Agglutination tests are employed to identify and differentiate bacteria based on their surface antigens. For example, the Widal test is used for the diagnosis of typhoid fever.

• 4. Pregnancy Tests: Pregnancy tests often use agglutination reactions. They detect the presence of human chorionic gonadotropin (hCG), a hormone produced during pregnancy.

• 5. Autoimmune Diseases: Agglutination tests are used in the diagnosis of autoimmune diseases. For example, the rheumatoid factor test detects the presence of autoantibodies present in the serum of individuals with rheumatoid arthritis.

• 6. Immunohematology: Agglutination reactions are essential in crossmatching for blood transfusions. Compatibility between donor and recipient blood is determined by observing agglutination reactions to avoid transfusion reactions.

• 7. Pathogen Detection: Some infectious diseases are diagnosed using agglutination reactions. For example, the latex agglutination test is employed for the rapid detection of certain bacterial and viral infections.

The agglutination reaction provides a rapid and visually detectable result. Thus it is a valuable tool in various diagnostic and research applications in clinical laboratories and healthcare settings.

Principle and applications of precipitation and flocculation

A combination of antibodies and antigens an optimal ratio form visible aggregates or precipitates. If the precipitate remains suspended as floccules in the reaction medium then it is called flocculation. This process is often used in laboratory settings for diagnostic and research purposes.

• 1. Binding site: Both antigens and antibodies have multiple binding sites to bind with each other.

• 2. Prozone and Postzone Effects: The prozone effect occurs when there is an excess of either antibodies or antigens. This prevents the formation of a visible precipitate. The postzone effect occurs when there is an insufficient concentration of either antibodies or antigens. This leads to incomplete precipitation.

• 3. Equilibrium Zone: The "equilibrium zone" has the concentration of antigen and antibody in balance. This results in the formation of antigen-antibody complexes. The concentration of antigen-antibody complex in the equilibrium zone is at its maximum.

• 4. Optimal Proportions: Precipitation reactions require an optimal ratio of antibodies to antigens. This ensures that each antigen molecule is bound by multiple antibodies. This promotes the formation of stable precipitates.

• 5. Solubility of Complexes: The formed antigen-antibody complexes should have limited solubility. This develops a stable precipitate. This is influenced by factors such as the size of the antigen and antibody molecules

• 6. Antigen-antibody reactions to produce precipitates occur either in solution or a gel.

Applications of Precipitation Reactions in Antigen-Antibody reactions:

Antigen-antibody reactions to produce precipitates occur either in a solution or a gel.

A. Precipitation Reactions in antigen-antibody reactions in solution

B. Precipitation Reactions in antigen-antibody reactions in gel

1. Immunodiffusion: Immunodiffusion techniques use the principle of precipitation for the identification and quantification of antigens and antibodies. Diffusion of antigens and antibodies towards

each other in a gel matrix leads to the formation of visible precipitin lines.

2. Radial Immuno diffusion: This technique involves placing an antigen-containing sample in a well within an agar gel containing antibodies. The antigens diffuse outward and react with the antibodies. This forms a visible ring of precipitation whose diameter is proportional to the antigen concentration.

Applications in Diagnostics: Precipitation reactions are used in various diagnostic tests. This includes tests for autoimmune diseases, infectious diseases, and the determination of blood group antigens

Principle and applications of flocculation reactions.

Flocculation is the aggregation or clumping of particles. It is a result of the interaction between antigens and antibodies. This phenomenon has several principles and applications, particularly in the field of immunology.

Principle of flocculation reactions in antigen-antibody reactions

The principle of flocculation reactions in antigen-antibody interactions is the formation of visible aggregates or clumps. These aggregates are called floccules. The following is the breakdown of the principle:

• 1. Antigen-Antibody Interaction: In an antigen-antibody reaction, specific antigens and antibodies interact with each other. This interaction forms antigen-antibody complexes.

• 2. Flocculation Agent: A flocculation agent is a multivalent cation such as calcium or magnesium ions. These ions are introduced into the system. These ions help to neutralize the charge on the antigen-antibody complexes.

• 3. Neutralization of Charges: The flocculation agent neutralizes the charges present on the surface of the antigen-antibody complexes. This neutralization allows them to come close.

• 4. Formation of Floccules: The antigen-antibody complexes aggregate and form visible clumps or floccules. These floccules are insoluble or poorly soluble and can be observed macroscopically.

• 5. Precipitation: Flocculation leads to the precipitation of the antigen-antibody complexes. Precipitation is the settling of these complexes out of solution due to their increased size and reduced solubility.

Applications of flocculation reactions

Flocculation reactions in antigen-antibody interactions have various applications in medical diagnostics, research, and industrial processes.

• 1. Immunoassay: Agglutination tests are widely used in clinical laboratories for the rapid detection of various infectious agents (e.g., bacteria, viruses), hormones, and biomarkers. For example, the Widal test for typhoid fever diagnosis utilizes flocculation reactions to detect antibodies against Salmonella typhi antigens.

• 2. Blood Typing: ABO and Rh blood grouping are commonly performed using agglutination assays. In these tests, antibodies against A, B, and Rh antigens cause red blood cells to agglutinate. This leads to visible flocculation that determines the blood group.

• 3. Serological Testing: Flocculation reactions are used in serological assays for infectious diseases. For example: the Venereal Disease Research Laboratory (VDRL) test and Rapid Plasma Reagin (RPR) test utilize flocculation to detect antibodies against Treponema pallidum antigens. It is used to diagnose syphilis.

• 4. Pregnancy Test: Some pregnancy tests rely on the agglutination of hCG (human chorionic gonadotropin) coated latex particles by anti-hCG antibodies present in urine or serum. The formation of visible aggregates or floccules indicates a positive result.

Dr. Pramila Singh