Cathelicidins are a family of host defence peptides that are known to selectively alter innate immunity in response to infection and other changes in immune status. A study in this issue of the European Journal of Immunology elucidates a new role for mouse cathelin-related antimicrobial peptide in the adaptive immune response by clearly demonstrating for the first time that a cathelicidin can alter T-cell-dependent activation of the humoral response in vivo and thus modulate the activities of both B and T lymphocytes.