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The study of blood serum. Following invasion of an animal body by foreign materials (antigens) the serum contains antibodies produced to combat the antigens. Serum containing antibodies produced against a particular kind of antigen is termed antiserum. Each antigen stimulates the production of a specific antibody active against that antigen alone or particles very like it. The specificity of the immune response has been exploited in the identification of viruses and in chemotaxonomy, where it is used to compare protein extracts from different plant species. This field of plant taxonomy is termed serotaxonomy. A laboratory animal, usually a rabbit, is injected with a plant protein extract. A few weeks later when the animal has produced sufficient antisera, a blood sample is taken. The antisera is then mixed in vitro with antigens from the same species used to raise the antisera (the homologous species extract) and subsequently against antigens from a range of other species. The degree of similarity between antiserum and antigen is measured by the turbidity produced in a liquid by the antibody-antigen precipitation reaction. The turbidity should be greatest when the antisera is mixed with the homologous species extract. Antigens from different species will produce varying amounts of precipitation, the more similar species producing greater turbidity than the less similar species.
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