Vaccination damage: Weakened polio viruses, for example, can mutate back into a virulent strain and cause infantile paralysis.

Vaccinia virus: The vaccinia virus, a cowpox virus, was the very first effective vaccine. It causes a very weak infection in humans which produces resistance to the human smallpox virus (variola).

Variability: The variability of a protein results from the differences in the amino acid sequences of various variants of this protein. The most highly variable proteins we know are the antibodies and T cell receptors.

Variable region: An amino-terminal domain which results during the maturation of lymphocytes from recombination of the V, D and J gene segments is called the variable region or V region.

Vesicles are bubbles surrounded by a membrane in the cytoplasm of a cell.

V gene segments: The V gene segments contain the information for the first 95 amino acids of the variables domains of the immunoglobulins and T cell receptors. A V gene segment must bind to a J or a DJ gene segment into a complete exon for a V domain before a functioning receptor chain can be expressed. The variable or V region of a receptor chain pairs with another V region or V domain to create a complete immunoglobulin or a complete T cell receptor.

Viruses are particles with a genome of nucleic acid which can reproduce only inside living cells, as they do not have their own metabolism.