Examples of MRI cell tracking labels include superparamagnetic iron oxide nanoparticles and 19F-containing compounds such as perfluorocarbons. The advantage of using cellular MRI to track cells is that a non-radioactive tracer is used that is suitable for longitudinal studies. MRI offers 3D imaging with exquisite soft tissue contrast. The additional advantage of using 19F over iron-based agents is that 19F is a positive, easily quantifiable contrast cell labelling agent. Furthermore, there is no 19F-background MRI signal in humans whereas MRI involving iron can be confounded by different imaging artifacts and it is a negative contrast agent that is difficult to accurately quantify. The investigators are particularly interested in whether immune cells arrive in secondary lymphoid organs such as lymph nodes.