These series of methods are assays by utilizing specific viral antigen to detect antibodies in patients’ serum, including IgM antibodies detection and IgG antibodies measurement. The IgM antibodies disappear in several weeks, whereas the IgG antibodies persist for many years. Establishing the diagnosis of a viral infection is accomplished serologically by demonstrating a rise in antibody titer to the virus or by demonstrating antiviral antibodies of the IgM class. The methods used include the neutralization (Nt) test, the complement fixation (CF) test, the hemagglutination inhibition (HI) test, and the immunofluorescence (IF) test, passive hemagglutination, and immunodiffusion.
A. Neutralization Assays
During infection or cell culture, virus can be inhibited by its specific antibody and loss the infectivity, this kind of antibody is defined as neutralization antibody. Neutralization assays is to detect the neutralization antibody in patients’ serum.
B. Complement Fixation Assays
The complement fixation assay can be used to look for the presence of specific antibody or antigen in a patient's serum. The test utilizes sheep red blood cells (SRBC), anti-SRBC antibody and complement, along with specific antigen (if looking for antibody in serum) or specific antibody (if looking for antigen in serum).
C. Hemagglutination Inhibition Assays
If the concentration of virus in a sample is high, when the sample is mixed with RBCs, a lattice of viruses and RBCs will be formed. This phenomenon is called hemagglutination. If antibodies against the hemagglutinins are present, hemagglutination will be prevented. During the hemagglutination inhibition test, serial dilutions of serum are mixed with a known amount of virus. After incubation, RBCs are added, and the mixture is left to sit for several hours. If hemagglutination is inhibited, a pellet of RBCs forms at the bottom of the tube. If hemagglutination is not inhibited, a thin film is formed.