Serology forms the mainstay of viral diagnosis. This is what happens in a primary humoral immune response to antigen. Following exposure, the first antibody to appear is IgM, which is followed by a much higher titre of IgG. In cases of reinfection, the level of specific IgM either remain the same or rises slightly. But IgG shoots up rapidly and far more earlier than in a primary infection. Many different types of serological tests are available. With some assays such as EIA and RIA, one can look specifically for IgM or IgG, whereas with other assays such as CFT and HAI, one can only detect total antibody, which comprises mainly IgG. Some of these tests are much more sensitive than others: EIAs and radioimmunoassays are the most sensitive tests available, whereas CFT and HAI tests are not so sensitive. Newer techniques such as EIAs offer better sensitivity, specificity and reproducibility than classical techniques such as CFT and HAI. The sensitivity and specificity of the assays depend greatly on the antigen used. Assays that use recombinant protein or synthetic peptide antigens tend to be more specific than those using whole or disrupted virus particles.