Natural killer (NK) cell phenotypes have been reported to be implicated in the pathogenesis of multiple sclerosis (MS). Several studies have observed reduced peripheral numbers, reduced cytotoxic activity, and altered CD56Dim and CD56Bright NK cell phenotypes. In this thesis I have investigated NK cell cytotoxicity, calcium mobilisation and transient receptor potential melastatin 3 (TRPM3) surface expression. NK cell cytotoxic activity and calcium signalling were examined in CD56Dim and CD56Bright NK cells before and after stimulation with ionomycin, pregnenolone sulphate, 2-aminoethoxydiphenyl borate and thapsigargin. Purified NK cells were labelled with antibodies to determine TRPM3, CD69 and CD107a surface expression using flow cytometry. Twenty-two people with MS and 22 healthy controls were recruited for this project. Twelve of the 22 MS cases previously received alemtuzumab and the remaining 10 were on no medication. TRPM3 was significantly increased in untreated MS compared with healthy controls and treated MS (p = 0.034). There was a significant decrease in CD69 surface expression on CD56Dim phenotype NK cells for untreated MS (p = 0.031) and treated MS (p = 0.036) compared to controls. Calcium mobilisation in CD56Bright NK cells and to a lesser extent CD56Dim NK cells between healthy controls, treated and untreated MS was also altered. In conclusion, these investigations suggest variations in TRPM3 expression and calcium mobilisation of NK cells may be implicated in the pathogenesis of MS. Further investigation is required to determine the mechanism by which alemtuzumab alters calcium signalling in NK cells. Neuromyelitis optica spectrum disorder (NMOSD) is an inflammatory disease of the central nervous system associated with antibodies to aquaporin-4 (AQP4) which has distinct clinical, radiological and pathological features, but also has some overlap with MS and myelin oligodendrocyte glycoprotein (MOG) antibody associated disease. Early recognition of NMOSD is important because of differing responses to both acute and preventive therapy. Magnetic resonance imaging (MRI) has proven helpful in this process. Key MRI clues to the diagnosis of NMOSD are longitudinally extensive lesions of the optic nerve (more than half the length) and spinal cord (3 or more vertebral segments), bilateral optic nerve lesions and lesions of the optic chiasm, area postrema, floor of the IV ventricle, periaqueductal grey matter, hypothalamus and walls of the III ventricle. Other NMOSD specific lesions are denoted by their unique morphology: heterogeneous lesions of the corpus callosum, "cloud-like" Gd-enhancing white matter lesions and "bright spotty" lesions of the spinal cord. The specificity of other lesions described in NMOSD, including linear periventricular periependymal lesions and 'patch' subcortical white matter lesions, are less certain. The use of advanced MR imaging techniques is yielding further useful information about focal degeneration of the thalamus and optic radiation in NMOSD and suggests that paramagnetic rim patterns and changes in normal appearing white matter are specific to MS. MR imaging is crucial in the early recognition of NMOSD and in directing testing for AQP4 antibodies and guiding immediate acute treatment decisions. Increasingly, MR imaging is playing a role in diagnosing seronegative cases of NMOSD. NMOSD and MS are inflammatory diseases of the CNS. Overlap in the clinical and MRI features of NMOSD and MS means that distinguishing these conditions can be difficult. With the aim of evaluating the diagnostic utility of MRI features in distinguishing NMOSD from MS, I have conducted a cross-sectional analysis of imaging data and developed predictive models to distinguish the two conditions. NMOSD and MS MRI lesions were identified and defined through a literature search. AQP4 antibody seropositive NMOSD cases were collected together with age- and sex-matched MS cases. MRI of orbits, brain and spine were reported by at least two blinded reviewers. MRI brain or spine was available for 166/168 (99%) of cases. Longitudinally extensive (odds ratio [OR]=203), "bright spotty" (OR 93.8), whole (axial; OR=57.8) or gadolinium (Gd) enhancing (OR=28.6) spinal cord lesions, bilateral (OR=31.3) or Gd-enhancing (OR=15.4) optic nerve lesions, and nucleus tractus solitarius (OR=19.2), periaqueductal (OR=16.8) or hypothalamic (OR=7.2) brain lesions were associated with NMOSD. Ovoid (OR=0.029), Dawson's finger (OR 0.031), pyramidal corpus callosum (OR=0.058), periventricular (OR=0.136), temporal lobe (OR=0.137) and T1 black hole (OR=0.154) brain lesions were associated with MS. A score-based algorithm and a decision tree determined by machine learning accurately predicted more than 85% of both diagnoses using first available imaging alone. I have confirmed NMOSD and MS specific MRI features and combined these in predictive models that can accurately identify more than 85% of cases as either AQP4 seropositive NMOSD or MS. NMOSD is associated with significant morbidity and mortality. Several therapies have been recommended for NMOSD and more recently clinical trials have demonstrated efficacy for three monoclonal antibody therapies. I present an observational study of treatment response in NMOSD. This was a retrospective, unblinded, observational study of treatment efficacy of rituximab, traditional immunosuppressive therapy and MS therapies (used inadvertently) in people with AQP4 antibody positive NMOSD. Treatment efficacy was assessed using annualised relapse rate (ARR), time to first relapse and Expanded Disability Status Scale scores. Complete relapse and treatment data were available for 43/68 (63%) of AQP4 antibody positive NMOSD cases covering 74 episodes of treatment. In a time to first relapse analysis rituximab showed a risk ratio of 0.23 (95% CI 0.08 - 0.65) when compared with no treatment and there was a nonsignificant reduction in ARR of 35% compared to pre-treatment. The MS therapy, β-interferon (p = 0.0002), and cyclophosphamide (p = 0.0034) were associated with an increased ARR compared to pre-treatment. A lower final EDDS was seen for Rituximab (median 4.0 [range 0.0 - 7.0]; p = 0.042) and traditional immunosuppressive therapy (median 4.0 [range 0.0 - 8.0]; p = 0.016) compared to β-interferon (median 6.0 [range 4.0 - 7.5]). These data provide additional support for the use of rituximab in preference to traditional immunosuppressive agents and MS disease modifying therapies as first line treatment of NMOSD.