An antigen that can trigger an immunological response is known as an immunogen. The production of antibodies is reliant on a humoral immune response, which is carried out by immune cells that identify a molecule as foreign. When an immunogen is injected alongside an adjuvant, the host's immune system is pushed to mount a particular immunological response, producing antibodies against the immunogen. Antigens less than 20 kDa (200 amino acids) are often not immunogenic. They must be attached to a carrier protein. The concentration of the injected antigen has an impact on immunogenicity in addition to overall size . The inoculation volume has to be more concentrated the lower the antigen's immunogenicity.