Vaccine - a planned exposure to an antigen in order that memory B lymphocytes can retain a memory for it. In practice the organism carrying the antigen is either killed or modified so that it does not cause the disease. When encountered again, the antigen is recognised and there will be a rapid production of antibodies. For example smallpox, polio, measles. Influenza vaccines are less effective as new strains of the virus are always occurring which do not have recognisable antigens.      Vacuole- a sac-like structure within a cell lined by cell membrane, containing material ingested by phagocytosis.     Vasoconstriction - a reduction in the diameter of small arteries (arterioles) which is caused by constriction of the smooth muscle fibres in the wall of the arteriole. Vasoconstriction is an important method of increasing the blood pressure. In local areas of damage it prevents blood loss by haemostasis. Local vasoconstriction can be caused by nerve impulses to the smooth muscle from the sympathetic nervous system, by locally released prostaglandins, serotonin and Epinephrine.     Vasodilation - an increase reduction in the diameter of small arteries (arterioles) which is caused by relaxation of the smooth muscle fibres in the wall of the arteriole. While vasoconstriction prevents blood loss in damaged tissues, vasodilation follows in order to allow the blood flow to slow down and clotting factors and leucotrines to seep into the damaged tissue. Local vasodilation can be caused by nerve impulses to the smooth muscle from the parasympathetic nervous system, and by locally released bradykinins.     Visco-elastic - a property of a material which combines elasticity and viscosity. The suspension of a car and the periodontal ligament of the tooth are examples of visco-elastic support. Elasticity refers to the return of a material to its original shape after being stretched or compressed. Viscosity is the resistance of a fluid to flowing fast.     Viscous - a liquid which has a high viscosity, or resistance to flow.