Hi Bill,

The hapten is too small on its own to be immunogenic. There is a rule about molecular weight and immunogenicity. Hapten can also often not a protein, which means it can not stimulate t cells, as they only recognize peptides ( in the context of mhc). However, when conjugated to a carrier protein, the complex is now large enough to be immunogenic; the carrier portion serves to activate th cells and b cells where as the happen can now stimulate the b cells. Sometimes also antibodies are raised that bind to a neo epitope formed by the conjugated complex.

So if you are immunizing using a happen carrier complex, you must always boost with the same carrier conjugate as what you used in the initial dose. This is called carrier effect.. To eliminate unwanted noise, you the screen the ensuing hypridomas colonies using the happen conjugated to a different epitope. Thus ensures that you only measure activity specific for the happen.

Thanks Lora

The hapten-carrier complex induces antibody formation, which the unbound hapten cannot, and so becomes immunogenic (capable of eliciting an immune response). The hapten then reacts with the antibodies that have been produced against it, causing an immunological or allergic response.