STEP THREE: Assays Infectivity Local lesion – plants; around infection sites, cells die (apoptosis) Plaque – animal & bac; more plaques, more virus End-point titration – dilution; LD50 > 50% animals die Immunological – Ab based (plants don’t make Ab’s; no bone marrows) Neutralizing test – unknown virus determined by antisera of known virus > only neutralizing antibodies can neutralize infectivity Immunofluorescence – antibodies cross-linked to fluorescence dyes Radio-immunoassay – the isotope of iodine fixed on Ab’s OR Ag’s Immuno-diffusion – Ab-Ag diffuse & form white precipitate in agar Haemolysin & Haemoagl’n – blood cells stick together or lyse by virus Complement fixation 1. KNOWN amount of complement added to Ag (virus) & Ab (mono, poly) > Ag-Ab complex consume complement 2. Sheep blood cells & anti-sheep cell serum added to the initial mixture > NON-FIXED complement is consumed to lyse sheep blood cells > red color indicates the amount of non-fixed complement
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