Lab Techniques for Immune

Antigen-Antibody Reactions in the Laboratory

Introduction to Serologic Tests

Serologic tests are essential diagnostic tools in medical microbiology and immunology, used to detect the presence of antibodies or antigens in blood or tissue. These tests rely on the specific binding between an antibody and its corresponding antigen, and results are often quantified as a titer, which is the highest dilution of a specimen that still yields a positive reaction.

• Antibody detection: Used to identify the presence of a specific antigen in blood or tissue.

• Antigen detection: Used to identify the presence of a specific antibody in serum.

• Titer: Indicates the concentration of antibody or antigen present.

Typical applications:

• Diagnosing infectious diseases (when culturing the organism is not feasible)

• Diagnosing autoimmune diseases (detecting antibodies against self-antigens)

• Determining blood type and HLA type (detecting ABO antigens or HLA epitopes)

Types of Diagnostic Tests

Agglutination

Agglutination tests detect the clumping of cells or beads due to the formation of antigen-antibody complexes. This method is commonly used in blood typing and in the detection of certain viruses and non-red blood cell antigens (hemagglutination test).

• ABO blood typing: Red cells agglutinate if they bear antigens for the test antibody.

• Hemagglutination test: Used to detect viral antigens or antibodies.

Example: In ABO blood typing, the presence or absence of agglutination indicates the blood group of the individual.

Precipitation

Precipitation tests detect the formation of visible precipitates when antigen-antibody complexes form. These can be measured in solution or in agar.

• Nephelometry: Measures the amount of antibody of a given class (e.g., IgM, IgG, IgA) in serum by detecting the amount of precipitate formed.

• Diffusion in agar (Ouchterlony test): Antibody and antigen diffuse toward each other in agar, forming lines of precipitation if they react.

Example: Anti-IgM antibody reacts with IgM in serum; more IgM results in more precipitate.

Labeling Antibodies and Antigens

Labeling techniques enhance the detection of antigen-antibody reactions by attaching a detectable marker to the antibody or antigen.

Radioimmunoassay (RIA)

• Uses radioactive labels to determine the concentration of antigens in serum.

• RAST assays for IgE that reacts with a specific allergen.

Enzyme-Linked Immunosorbent Assay (ELISA)

• Uses enzymatic labeling of antibodies.

• Highly sensitive, non-radioactive, and can be automated.

• Detects either antigens or antibodies in serum.

• Enzyme linked to anti-Ig antibody reacts with substrate to produce a detectable color.

Key features of ELISA:

• Specific antigen-antibody binding

• Detects antigens or antibodies

• Very sensitive (detects small amounts)

• Non-radioactive

• Can be quantitative

• Usually performed in a 96-well plate

Types of ELISA

Type	Description
Direct ELISA	Antigen is immobilized, detected by enzyme-linked antibody.
Indirect ELISA	Antigen is immobilized, detected by primary antibody and enzyme-linked secondary antibody.
Sandwich ELISA	Capture antibody immobilized, antigen binds, detected by enzyme-linked antibody.
Competitive ELISA	Antigen in sample competes with labeled antigen for antibody binding.

Example: Indirect ELISA is used to detect antibodies produced by mouse hybridomas against chicken ovalbumin (OVA).

Colorimetric Reaction Catalyzed by HRP

Horseradish peroxidase (HRP) catalyzes the conversion of tetramethylbenzidine (TMB) to a colored product, indicating the presence of the target antigen or antibody.

• $\text{TMB} + H_2O_2 \xrightarrow{HRP} \text{TMB oxidized (blue complex)} \xrightarrow{H_2SO_4} \text{yellow}$

Affinity Chromatography Using Antibodies

Antibodies can be attached to beads to construct affinity columns for protein purification. These antibodies may recognize natural epitopes or artificially fused tags on proteins.

• Tagged proteins bind to antibody-coated beads.

• Elution with buffer releases the bound protein.

Immunofluorescence

Immunofluorescence uses fluorescent dye labeling of antibodies to detect antigens or antibodies in cells or tissue sections.

• Visualized under UV light on a microscope slide.

• Detects antigens and proteins directly (direct method) or via secondary antibodies (indirect method).

• Can be used for diagnostic purposes similar to ELISA.

Confocal Microscopy

Confocal microscopy uses lasers and special detection systems to scan layers in a cell, assembling pixels into 2D or 3D images. Proteins are often tagged with fluorescent antibodies for identification.

• Allows tagging of several targets with different colors using lasers of different wavelengths.

Flow Cytometry (Fluorescence Activated Cell Sorting, FACS)

Flow cytometry separates antibody-labeled cells from unlabeled ones in a population using a laser detection system. It is used to analyze and sort cells based on their fluorescence characteristics.

• Cells pass through a laser beam; fluorescence is detected and measured.

• Allows sorting and analysis of cell populations.

Complement Fixation Test

The complement fixation test assays for antibody to a particular antigen in a patient's serum. It is based on the ability of antigen-antibody complexes to fix complement, which can then be detected by lysis of indicator cells.

• Positive reaction: Complement is fixed, indicator cells are not lysed.

• Negative reaction: Complement is not fixed, indicator cells are lysed.

Western Blot (Immunoblot)

The Western blot is used diagnostically and in research to detect specific proteins or antibodies in a sample. It is commonly used to confirm HIV-positive diagnoses by detecting serum antibodies to the virus.

• Proteins are separated by gel electrophoresis, transferred to a membrane, and detected using labeled antibodies.

ABO Blood Group Immunology

ABO Blood Group Antigens

ABO blood group antigens differ in their terminal sugars on red blood cells (RBCs). The A gene encodes an enzyme that adds N-acetylglucosamine, the B gene adds galactose, and type O individuals have neither allele, carrying only the H antigen.

• A and B alleles are dominant over O, but co-dominant with each other.

Blood Group	Antigen Present	Antibody Produced
A	A antigen	Anti-B
B	B antigen	Anti-A
AB	A and B antigens	None
O	H antigen	Anti-A and Anti-B

Blood Transfusion Compatibility

Antibodies against antigens in an unmatched transfusion will agglutinate and destroy donor RBCs. Type O is a universal donor, while type AB is a universal recipient.

Donor	Recipient	Compatible?
O	Any	Yes
AB	AB	Yes
A	A, AB	Yes
B	B, AB	Yes

Hemolytic Disease of the Newborn (Rh Incompatibility)

Hemolytic disease of the newborn occurs when an Rh-negative mother carries an Rh-positive fetus. Maternal antibodies against the Rh antigen can cross the placenta and destroy fetal RBCs.

• Susceptible mothers can be treated with Rho-Gam antibody, which blocks the Rh antigen on fetal cells.

Father	Mother	Child	Risk of Hemolytic Disease
Rh+	Rh-	Rh+	Yes
Rh-	Rh-	Rh-	No

Additional info: These diagnostic methods are foundational in clinical immunology and transfusion medicine, but are not directly relevant to organic chemistry topics such as reaction mechanisms, spectroscopy, or molecular structure. However, the principles of antigen-antibody binding and detection methods may intersect with biochemistry and molecular biology.