 The overall goal of this double-blind Phase II study is to evaluate the safety, efficacy and biological mechanisms of action of Intravenous Immunoglobulin IVIg in the treatment of mild to moderate stage Alzheimers disease AD. IVIg contains antibodies against the amyloid beta protein that is the central component of the AD senile plaque. It is hypothesized that IVIg treatment will reduce the levels of beta amyloid in the brain and improve cognitive abilities relative to placebo. A total of 24 patients with mild to moderate AD capable of giving informed consent will be randomly assigned to receive either IVIg 16 patientsor saline placebo 8 patients for six months. This study includes comparison of four dosing regimens of IVIg. Cognitive, behavioral and functional measures will be collected at baseline, three months and six months of treatment. Plasma samples will be collected before and after infusions. Subjects will undergo a lumbar puncture before and after the six months of treatment for cerebrospinal fluid CSF biomarker analyses. In addition, Positron Emission Tomography PET imaging substudies will be performed at two time points during the study. Following the initial 6 month placebo-controlled period, all participants have the opportunity to receive IVIg for an additional 12 month period in an extension study. 