The hydatid cyst (metacestode) of Echinococcus granulosus is the ethiological agent of cystic hydatid disease (CHD) in humans and domestic ungulates. Antigen B (AgB) was initially identified as a thermostable immunodominant lipoproteic component of the hydatid cyst fluid, and, later, characterized as a 120-160 kDa multimeric protein. Here, we review the recent advances on the AgB structural, functional and immunological characterization, summarizing results obtained by our group and other laboratories. AgB has been described as a polymer of related 8 kDa subunits, produced both by the hydatid cyst’s germinative layer and protoscoleces (preadult forms). At least five AgB 8 kDa subunit encoding genes are present in the E. granulosus genome, forming a multigene family with extensive polymorphisms among E. granulosus isolates or strains. Protein studies suggest that native AgB is constituted by at least two different types of subunits. This AgB heteropolymeric nature, along with the availability of a repertoire of several subunit enconding genes, suggests the interesting possibility of AgB subunit composition variation during infection, a possible evasion mechanism from host immune response. There are evidences that AgB elicits a nonprotective Th2 cell response, and different strategies, from epitope mapping to molecular modeling, are being used to elucidate determinant aspects of its immunodominance.  Native AgB subunits have been described as important antigens for CHD immunodiagnosis,  with  variable  results  depending on the antigen source or laboratory. CHD immunodiagnosis approaches have been improved by the use of recombinant AgB subunits and synthetic peptides derived from AgB immunodominant epitopes.