The investigators propose evaluating patients with B cell non-Hodgkin's lymphoma treated with rituximab within the past 2 years with baseline immunoglobulin levels and vaccine responses to polysaccharide (pneumococcus, meningococcus) and peptide (tetanus, diphtheria) antigens. Patients with impaired vaccine responses may benefit most from immunoglobulin prophylaxis and will be proactively started on 20% subcutaneous replacement therapy. This study is novel in that it will stratify patients according to their humoral response to polysaccharide and peptide vaccines, and will proactively initiate therapy with the new 20% subcutaneous immunoglobulin in those with impaired humoral response rather than starting it after infections occur. This will potentially lead to decreased infections and improved quality of life.