Glycoconjugate vaccines have been very successful at reducing disease incidence from a number of pathogenic bacteria. The aim of this study is to quantitatively assess the relationship between conjugate size and immune response and improve the understanding of how saccharide loading and chain length influence immune response. Firstly, various commercial conjugates with a tetanus toxoid (TT) carrier protein were examined. An analysis of structure-antibody recognition relationships in nine licensed polysaccharide(PS)-TT conjugate vaccines was performed. The commercial conjugates had a wide range of molecular masses from 1.8Ã10^6 g/mol to larger than 20Ã10^6 g/mol. TT epitope recognition was measured using four monoclonal antibodies. A correlation was found between accessibility of tryptophan (Trp) side chains and PS loading, suggesting that a higher level of conjugated PS does not interfere with toxoid accessibility. Different levels of TT Trp side-chain and epitope accessibility were related to saccharide type, despite the conjugates being independently manufactured and using different conjugation chemistries. To further examine the link between conjugate size and immunogenic response, a panel of four glycoconjugate meningococcal serogroup C (MenC) vaccines of different size (molar mass) and PS chain length was prepared. The panel consisted of conjugates of 191,500 to 2,348,000 g/mol with hydrodynamic radii (Rh(v)h) from 12.1 nm to 47.9 nm formed through conjugation of a monomeric TT to MenC PS of varying chain lengths. Sized MenC PSs of between 24,070 and 441,800 g/mol were obtained through altering hydrolysis conditions and fraction collection. The two larger panel vaccine conjugates produced higher anti-MenC IgG titres, with IgG1>IgG2b. Larger vaccine conjugate size similarly stimulated greater T cell proliferative responses in an in vitro recall assay. In vitro, innate cytokine responses (RANTES, MIP-2 and MIP-1a) were also greater with larger conjugates. In conclusion, larger MenC-TT conjugate sizes correlates with greater humoral, cellular and innate murine immune responses, likely to translate into superior vaccine protection. Commercial manufacturers could choose to produce vaccines to a required size or with selected structural properties. This is the first study looking at a quantitative assessment of MenC-TT conjugate size and the effect of structural properties on the immune response generated. Further work could be undertaken to determine an optimum vaccine size, for MenC conjugates, other conjugate types and different vaccine types.