Abstract: The invention provides methods and uses for stimulating, inducing, increasing or enhancing Tfh (follicular helper) cell differentiation, germinal center B cell development or an antibody response, and an immune response by administering activin or a subsequence thereof, or an activator of activin receptor, in an amount effective to stimulate, induce, increase or enhance Tfh (follicular helper) cell differentiation, germinal center B cell development or an antibody response, and an immune response, respectively. Also provided are peptides comprising or consisting of a subsequence of human activin sequence, in which the activin sequence or subsequence stimulates, induces, increases or enhances development of Tfh (follicular helper) cell differentiation, germinal center B cell development, an antibody response, an immune response and/or an immune response induced by a vaccine.