Hypothetical immunogenetic universes containing only three different classes of fundamental immunogenetic units (aggs, ants and antis) are given. An obligatory one one relation exists between these units, whereby definable many-many relations are obtained between genes, antigens and antibodies consisting of combinations of these fundamental units. Different serological universes (S-Y) are defined and found to require only a very small number of different fundamental units to account for most immunogenetic systems even at a fairly sophisticated level of knowledge. The article represents a first attempt towards a unifying concept of immunogenetic systems and serological reactions in general.