serotype

thumb|right|300px|Two serotypes 1a and 1b with antigens 2a and 2b on surface, which are recognized by two distinct [[antibodies, 3a and 3b, respectively]]

A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their shared reactivity between their surface antigens and a particular antiserum, allowing the classification of organisms to a level below the species. A group of serovars with common antigens is called a serogroup or sometimes serocomplex.