The rationale for hybridoma technology is the inherent properties of the mammalian immune system: the antibody response to a foreign antigen is clonal, committed, and heterogeneous. Although a given antibody producing cell will secrete an antibody of one, and only one, specificity regardless of subsequent exposure to another antigen, any given antigen will stimulate many independent antibody producing cells, resulting in the heterogeneous, polyclonal antibody response observed in immune serum (Fig. 1). If an individual antibody producing cell could be immortalized, it would then make homogeneous monoclonal antibody indefinitely. This is essentially the hybridoma process (Fig. 2). As shown by Kohler and Milstein in 1975 [1], when a lymphocyte secreting a specific antibody is fused with a myeloma cell the resulting hybrid shows dominant expression of two critical parental traits, specific antibody production (from the lymphocyte) and immortal growth (from the myeloma). The ability to immortalize specific antibody producing cells allows the dissection of the immune response and the production of antibodies with precisely defined characteristics. In turn, the ability to select such antibodies opens the door to novel applications in diagnostics, therapeutics, and separation technology.