The range of antibodies which an individual possesses, usually based on sampling from blood serum or saliva. Different strains of a pathogen can sometimes be distinguished by the different antibodies they induce in a host, or with which they can be made to react in vitro; thus the word serotype has also come to be applied to a particular strain (`the virulent serotype'). This is the more common clinical usage. The range of antibodies used to define a serotype obviously depends on those available to the researcher. Sometimes, as for measles, the presence of a known antibody within the serum of an individual correlates extremely well with the clinical observation that that individual is protected against any further infections. But sometimes, as for malaria, there is as yet no definite relationship between a given serotype and the presence of a functional immunity, which may make the use of the word serotype unhelpful when trying to distinguish between different parasites for the purposes of understanding transmission