Which of the following reactions in serology helps identify a virus?

The identification of a virus in serology predominantly relies on the reaction with specific antibodies. This method is based on the principle of immunology, where antibodies are proteins produced by the immune system in response to foreign pathogens, including viruses. When a virus is present, it can trigger the immune response, leading to the production of specific antibodies that target components of the virus.

In serological assays, samples are tested for the presence of these antibodies, which can indicate whether a particular virus is present in the sample. Techniques such as ELISA (Enzyme-Linked Immunosorbent Assay) and Western blotting leverage this interaction between antibodies and viral antigens for diagnosis. This specificity and sensitivity make antibody reactions a crucial tool for virus identification.

In contrast, a physical examination of the plant, observing color changes, or identifying nutrient deficiencies are non-specific methods that do not provide the same level of definitive identification for viral pathogens. While these methods can yield important information about plant health, they do not directly detect the presence of viruses or their specific antibodies.