This involves combining the antigen with the antibody and incubating the mixture. This has the effect of "clumping" the virus particles, sometimes when none have been seen by conventional electron microscopy (see Note 17), because of low concentration. If an excess of antibody is present each virus particle is surrounded by a halo of antibody molecules called "antibody coating" or "decoration" (1,6).
1. Using a variable volume, pipet dilute required specific antiserum to one-tenth of its normal precipitin titre (i.e., 1:100 for antiserum with a titre of 1:1000) in 0.06 M phosphate buffer, pH 6.5. Antiserum dilutions should be made fresh each time of use.
2. Squash approx 2-mm square of infected material (see Note 18) in 10-15 ^L of diluted antiserum on a glass slide and incubate in a humid container at room temperature for 15 min.
3. Hold a carbon-coated grid in forceps and touch to the mixed drop, wash with 20 drops of phosphate buffer, pH 6.5, 30 drops of distilled water, and three to five drops of 2% uranyl acetate, before draining with filter paper and drying.
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