What is an Immunogen?

Why Antigens Are Not Necessarily Immunogens

While all immunogens are antigens, not all antigens are immunogens. This is because some antigens are too small or difficult to bind to be easily detected by the immune system, subsequently preventing macrophages from collecting the antigen and activating B-cells. Without the activation of B-cells to produce specific antibodies that recognize the foreign antigen, there will be no humoral response. If this is the case, the antigen is not an immunogen. In contrast, immunogenic antigens are able to elicit a humoral immune response and have antibodies generated against it, leading to antibody development.

Carrier Proteins Make Small Antigens Immunogenic

By themselves, carrier proteins are easily detected antigens. The immune system will develop antibodies against these proteins very quickly so that they may be targeted by macrophages as foreign particles. By conjugating small target antigens to carrier proteins, most of the antibodies developed by the host will target the carrier, but enough antibodies will also target the conjugated antigen, successfully increasing the targets immunogenicity and allowing for antibody development. The excess antibodies against the carrier protein can be washed away by performing affinity purification, successfully isolating antibodies against the target of interest.

Common Types of Carrier Proteins

Carrier proteins are generally chosen for their large size and distance from the host evolutionarily. The bigger the carrier and the more foreign in structure it is, the more immunogenic it is, increasing the chances of successfully developing antibodies. Each carrier has benefits over the other; the correct choice depends on the nature of the antigen of interest, the host being used to develop antibodies, and the final application for the antibody.

KLH

Keyhole Limpet Hemocyanin (KLH)

Keyhole Limpet Hemocyanin (KLH) is a copper-containing oxygen carrier in the Keyhole Limpet sea snail that is around 390kDa. This massive mollusk protein is effective as an immunogen because of the complexity of its structure and massive size. The distance of this creature from humans on the evolutionary tree causes this immunogen to be immediately recognized as foreign. This is a tried and true carrier protein, and is the most recommended choice for conjugation.

BSA

Bovine serum albumin (BSA)

Bovine serum albumin (BSA) is a carrier protein in the sera of cows that is around 66kDa. It is highly abundant and great an eliciting an immune reaction. BSA can be manipulated to have many positive charges on it, which aids the process of conjugating an antigen to it. However, BSA can be used in other experimental applications, so it should only be used for antibody development if it will not be appearing anywhere else in latter applications.

OVA

Ovalbumin (OVA)

Ovalbumin is the main protein in egg white, and is the smallest suggested carrier protein; around 45kDa. The advantage that OVA provides over other immunogens is that it is soluble in dimethylsulfoxide (DMSO), a polar solvent. This makes it a great candidate for conjugation with antigens that are not soluble in aqueous solutions.

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