The invention concerns a pseudopeptide of at least 6 amino acids comprising at least a unit selected among the general formulae (I) and/or (II) wherein: R.sub.1, R.sub.2 and R.sub.3 each independently of one another represent a side-chain of amino acids and can be identical or different; X represents an oxygen or sulphur atom. The invention also concerns its synthesis process, a reagent containing it, a detection kit comprising such a reagent, a method for detecting an antigen or an antibody using said pseudopeptide, and antibody or and anti-idiotype and finally a therapeutic composition.