ELISALin Chengyu Bio 04 2010030007Experiment Date: 2012-03-12 Submitting Date: 2012-03-211Introduction1.1Background informationELISA (Enzyme-linked Immunosorbent Assay) is a solid-phase assay for antibodiesemploying ligands labeled with enzymes which is widely used for immunological assays.This technique can be applied to detect antigens or antibodies for qualitative orquantitative purpose. Since enzyme reactions are very well known amplificationprocesses, the signal is generated by enzymes which are linked to the detection reagentsin fixed proportions to allow accurate quantification.11.2Major principlesFigure 1 Schematic diagram of ELISA2Figure 2 Procedure of indirect ELISA3As shown in Figure 1 & 2, the general procedure of indirect ELSIA is to: incubate theplate well with antigen, wash off unbounded antigen, incubate with 1st antibody, washoff unbounded 1st antibody, incubate with labeled 2nd antibody, wash off unbounded 2ndantibody, incubate with enzyme substrate solution, and detect optical density or otherindex showing enzyme activity.2Experiment Operation2.1Antigen coating(1)Prepare an antigen solution in coating buffer (human IgG at 0.025mg/ml);(2)Pipette 200 μl antigen solution to each well (Row: B~G; Column: 2~10; Column 11is negative control without antigen) of the microtiter plate;(3)Incubate the plate at 37 ℃for 30 min;(4)Remove the antigen solution;(5)Wash each well with 200 μl with PBS-T for 3 times;(6)Block each well (Row: B~G; Column: 2~11) with 200 μl 0.5% BSA-PBS, andincubate the plate at 37 ℃for 30 min;(7)Remove the blocking solution;(8)Wash each well with 200 μl with PBS-T for 3 times.2.2Primary antibody reaction(1)Dilute the primary antibody (rabbit-anti-human IgG antiserum) in PBS-T fordifferent dilution (from 1:400 to 1:51,200 in 2-folds dilution);(2)Add 200 μl diluted antibody solution to each well following Table 1;Table 1 Scheme to add primary antibody(3)Incubate the plate at 37 ℃for 1 hour;(4)Remove the primary antibody solution;(5)Wash each well with 200 μl PBS-T for 3 times.2.3Application of secondary antibody(1)Dilute the peroxidase conjugated secondary antibody (Goat-anti-rabbit IgG-HRP) inPBS-T at the dilution of 1:20,000 and 1:40,000;(2)Add 200 μl secondary antibody solution to each well following Table 2;(3)Incubate the plate at 37 ℃for 1 hour;(4)Remove the secondary antibody solution;(5)Wash each well with 200 μl PBS-T for 3 times.2.4Substrate development(1)Add 200 μl substrate solution to each well (Row: B~G ,Column: 2~11);(2)Incubate for approximately 3 min;(3)Add 50 μl 2 M H2SO4 to each well to terminate the reaction;(4)Measure optical density at 490 nm.3Raw data and its processing3.1Raw data3.2Data processingSet Row B, C, and D as Group I, and Row E, F, and G as Group II. The processed datais shown in Table 4Table 4 Processed data: optical density of each groupSet different dilutions of primary antibody as x axis, optical density as y axis, drawFigure 3 to illustrate their relation.Figure 3 Relationship between optical density and dilutions of primary antibodyFor the reason that the curve cannot illustrate the relationship enough, change the x axis to nature logarithm of different dilutions of primary antibody. See Figure 4:Figure 4 Relationship between optical density and natural logarithm of dilutions of primary antibodyUsing linear fit for each group, we can figure out that two lines are approximately parallel.In the black curve in Figure 3, there is an oblivious point of inflection which corresponds with the dilution of 1:800. The curve after this point becomes flat, which indicates that the binding between antigens and primary antibodies is saturated in the dilution of 1:800 and higher. This data can suggest that in other immunoenzymatic experiment, the proper dilution of primary antibody will be around, and no higher than 1:800.What’s more, from the red line in Figure 4 we can figure out that the optical density hasa linear relation with natural logarithm of dilutions of the primary antibody.As for comparison between Group I and Group II, from Figure 3 we can figure out thatthe point of infection of blue curve, which corresponds with the dilution of 1:40,000, ison the left, about 1:1600.In Figure 4, the green line (1:40,000) is positioned lower than the red line (1:20,000),which is easy to understand. Lower concentration of secondary antibody means lessbinding with primary antibody during application of secondary antibody.4Results and discussion4.1Results(1)The optical density has an approximately linear relation with the natural logarithmof the dilutions of the primary antibody;(2)For secondary antibody in the dilution of 1:20,000, the proper dilution of primaryantibody is 1:800; for secondary antibody in the dilution of 1:40,000, primaryantibody is recommended to be 1:1600;(3)With the same dilution of antigen and primary antibody, higher concentration ofsecondary antibody will get a higher optical density;4.2Discussion(1)What is the significance of the negative control groups?I.The no primary antibody groups proved that there is no specific bindingbetween antigen and secondary antibody, and provided a background ofnon-specific binding between secondary antibody and antigen;II.The no antigen groups can provide a background of non-specific binding between primary antibody and BSA.(2)Why washing step is essential?Washing each well with PBS-T, which contains tween-20 as detergent, can wash offunbounded antigens and antibodies, including those non-specifically binding. Ifwashing step is omitted, the background index will be higher, and might causeinterference to the result.(3)Why blocking step is essential?After the antigen coating step, the surface of the well is not covered by antigenentirely, i.e. there is still some site leaving blank, which allows other proteins bindto them. Blocking step is to block those blank sites with non-specific bindingmaterial that will not cause interference to the experiment. Thus, the primaryantibody will only bind to the antigen coated in the first step, rather than coat on thesurface as well.(4)What’s the advantage of indirect ELISA comparing with direct ELISA?I.Indirect ELISA can amplify the optical density which we measure.Compared to direct ELISA, the number of secondary antibody binding tothe primary antibody is way larger than the number of primary antibodybinding to the antigen. Thus, optical density will be higher and easier tomeasure, which means a lower error;II.The secondary antibody contains HRP, which is essential for substrate development. Compared to direct ELISA, indirect ELISA need only onekind of antibody contains HRP to perform many kinds of experiment, ratherthan one antibody linked to enzyme for one experiment, which isinconvenient.5Reference【1】/wiki/ELISA【2】/post/9314400054【3】/indirect_elisa。
