Enzyme-linked Immunosorbent Assays (ELISAs) combine the specificity of antibodies with the sensitivity of simple enzyme assays, by using antibodies or antigens coupled to an easily assayed enzyme that possesses a high turnover number. ELISAs can provide a useful measurement of antigen or antibody concentration.

Sandwich ELISA Assays

To utilize this assay, one antibody (the “capture” antibody) is purified and bound to a solid phase typically attached to the bottom of a plate well. Antigen is then added and allowed to complex with the bound antibody. Unbound products are then removed with a wash, and a labeled second antibody (the “detection” antibody) is allowed to bind to the antigen, thus completing the “sandwich”. The assay is then quantitated by measuring the amount of labeled second antibody bound to the matrix, through the use of a colorimetric substrate. Major advantages of this technique are that the antigen does not need to be purified prior to use, and that these assays are very specific. However, one disadvantage is that not all antibodies can be used. Monoclonal antibody combinations must be qualified as “matched pairs”, meaning that they can recognize separate epitopes on the antigen so they do not hinder each other’s binding

Briefly, an unlabeled purified primary antibody is coated onto the wells of a 96 well microtiter plate. This primary antibody is then incubated with unlabeled standards and unknowns. After this reaction is allowed to go to equilibrium, conjugated immunogen is added. This conjugate will bind to the primary antibody wherever its binding sites are not already occupied by unlabeled immunogen. Thus, the more immunogen in the sample or standard, the lower the amount of conjugated immunogen bound. The plate is then developed with substrate and color change is measured.

Competitive ELISA Assays

Introduction to Antibodies - Enzyme-Linked Immunosorbent Assay (ELISA)

An assay for quantitating either antibody or antigen by use of an enzyme linked antibody and a substrate that forms a colored reaction product.

ELISPOT ASSAY

Immunoprecipitation

lenth.

Immunofluorescence

Immunofluorescence

The Technique

1. A mixture is prepared of

• radioactive antigen

(Because of the ease with which iodine atoms can be introduced into tyrosine residues in a protein, the radioactive isotopes 125I or 131I are often used.)

• antibodies against that antigen.

2 Known amounts of unlabeled ("cold") antigen are added to samples of the mixture. These compete for the binding sites of the antibodies.

3. At increasing concentrations of unlabeled antigen, an increasing amount of radioactive antigen is displaced from the antibody molecules.

4. The antibody-bound antigen is separated from the free antigen in the supernatant fluid, and

5. the radioactivity of each is measured.

6. From these data, a standard binding curve, like this one shown in red, can be drawn

7. The samples to be assayed (the unknowns) are run in parallel.

8. After determining the ratio of bound to free antigen in each unknown, the antigen concentrations can be read directly from the standard curve (as shown above).

Radioimmunoassay

RIA is highly sensitive technique for measuring antigen or antibody that involves competitive binding of radio-labelled Ag or Ab.

Separating Bound from Free AntigenThere are several ways of doing this.

•Precipitate the antigen-antibody complexes by adding a "second" antibody directed against the first. For example, if a rabbit IgG is used to bind the antigen, the complex can be precipitated by adding an antirabbit-IgG antiserum (e.g., raised by immunizing a goat with rabbit IgG). This is the method shown in the diagram above. •The antigen-specific antibodies can be coupled to the inner walls of a test tube . After incubation,

•the contents ("free") are removed; •the tube is washed ("bound"), and •the radioactive of both is measured.

•The antigen-specific antibodies can be coupled to particles, like Sephadex. Centrifugation of the reaction mixture separates

•the bound counts (in the pellet) from •the free counts in the supernatant fluid.

Radioimmunoassay is widely-used because of its great sensitivity. Using antibodies of high affinity it is possible to detect a few picograms (10−12 g) of antigen in the tube.

RIA has become a major tool in the clinical laboratory where it is used to assay •plasma levels of:

•most of our hormones; •digitoxin or digoxin in patients receiving these drugs; •certain abused drugs

•for the presence of hepatitis B surface antigen (HBsAg) in donated blood; •anti-DNA antibodies in systemic lupus erythematosus (SLE).

The technique of radioimmunoassay has revolutionized research and clinical practice in many areas, e.g.,

•blood banking •diagnosis of allergies •endocrinology

Ag can be adsorbed onto the inner surface of a plastic tube and test serum can then be added. If the serum contains auto-Abs specific for the bound Ag, it will bind to the Ag. A radioactively labeled secondary Ab (anti-human IgG) can then be added which attaches only to the Fc portion of IgG. The tube is washed and then the radioactivity in the tube is measured in a gamma counter. If the serum does not contain auto-Abs, then none of the radioactive IgG should bind, and there should be little radioactivity in the tube. However, small amounts of auto-Ab in the serum should lead to the binding of labeled anti-IgG, which can be detected in the gamma counter. RIAs are used to determine the presence of intrinsic factor (pernicious anemia), anti-DNA Abs (SLE), and antithyroglobulin IgG (thyroiditis). These tests can also be used to screen people who are considered at risk for a specific autoimmune disease.

The radioimmunoassay (RIA) is a very sensitive technique for detecting small quantities of auto-Abs in the serum of persons suspected of having autoimmune disease.

Flow CytometryFlow Cytometry is a powerful tool in research and clinical laboratory medicine for the diagnosis, isolation and study of specific cell types and disease conditions. Flow cytometry is defined as the measurement of the cellular and fluorescent properties of particles in suspension as they pass by a laser or other light source. These particles are, in most cases, cells. The measurements are represented by changes in light scattered, light absorbed, and light emitted by a cell as it passes by fixed detectors directed off the light source. From these measurements, specific populations and subsets within them are defined and even isolated physically using a dedicated cell sorter typically by manipulating cell charge (see Figure 1).

All light signals, whether from fluorescently labeled cells or from the beam scattered by unlabeled cells, are transferred to a computer and transformed into digital signals. These signals are displayed in channels along a histogram graph. The typical output is a single parameter graph of intensity versus the number of cells (see Figure 2) or two parameter histograms to distinguish subsets of cells.

Figure 1. Schematic Diagram of Flow Cytometry. Cells are sorted

according to their size and fluorescence.

Figure 2. Staining of human peripheral blood mononuclear cells (PBMC) using the T10B9 antibody (Cat. No. MAB1165). T10B9 recognizes a unique monomorphic antigenic determinant of the T cell receptor expressed on the majority of human T cells. Human PBMC were prepared by density gradient separation and stained with T10B9 at the concentration of 10 µg/mL, with subsequent staining using goat anti-mouse light chain antibodies labeled with FITC (closed line histogram). The open line histogram represents an isotype (mouse IgM) control. Data courtesy of Dr. C. Keever-Taylor, Medical College of Wisconsin, Milwaukee, WI.

                                                                                                  

Figure 4. Two dimensional histogram, displaying a scatter plot of white blood cells passing through a flow cytometer.

Uses for flow cytometry

Percentage of a total population of cells

Measuring antigen density within a populationof cells

Multiple antibodies can be used to assess severalcell surface antigens simultaneously

Clinical analysis (tumor characterization)

Usefulness :

A technique in which Ab used to stain a cell or tissue are labelled with an electron dense material and visualised with an electron microscope.