Regarding antibodies, monospecific and monovalent overlap in meaning; both can indicate specificity to one antigen, one epitope, or one cell type (including one microorganism species).
They are inserted into an animal, e.g. rabbit, which produces antibodies against the fragment.
[1] Recent research has led to the discovery that unstable hinged monospecific antibodies may engage in a process leading to a decrease in their apparent avidity/affinity.
This process, termed Fab arm exchange, has led to theories about the dissemination of viral infections in patients given monospecific IgG4 therapeutic antibodies.
Evidence is suggestive that this process is linked to the dissemination of PML in patients given Tysabri for MS.