Abstract: Provided is a method for activating CD4+ T cells using a polymer-based antigen complex. The method comprises the steps of bringing the polymer-based antigen complex into contact with B cells so that B cells process and present the antigen complex, and of bringing the B cells into contact with CD4+ T cells to activate CD4+ T cells. Also provided are a method for promoting the differentiation of CD4+ T cells into Tfh cells and Thl cells using the antigen complex, and a method for treating diseases by activating CD4+ T cells and/or promoting the differentiation of CD4+ T cells.