Vaccine Antigen Choice and the Immune Response It Elicits | AI Learning Guide

When you choose a vaccine antigen, you are choosing which immune response you want to train. If you pick a surface protein that antibodies can see on the intact virion, you train neutralizing antibodies that block the virus before it enters a cell. If you pick an internal protein that gets chopped up and displayed on MHC class I inside an infected cell, you train CD8 T cells that kill the infected cell. Most vaccines try to do both. But there is a catch: the surface protein that antibodies see is often the same protein that mutates during antigenic drift, so the vaccine must be updated. Internal proteins tend to be more conserved, so T cell responses may cover more strains, but they cannot stop the virus from entering a cell in the first place. That is the trade-off at the heart of antigen choice.

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Surface antigens and neutralizing antibodies

Neutralizing antibodies bind to the virion before it enters a cell and block attachment or fusion. The most effective target for neutralizing antibodies is therefore a surface protein that is exposed on the virion and essential for entry. Because the antibody must physically block the entry function, the antigen must present the same conformation it has on the native virion. This is why vaccine antigens are often expressed as stabilized trimers or on the surface of a particle: the goal is to preserve the epitopes that neutralizing antibodies recognize.

Internal antigens and T cell responses

Internal viral proteins are not exposed to antibodies on the intact virion, but they are processed inside infected cells and presented on MHC class I. A vaccine that delivers an internal antigen into the cytosol can therefore prime CD8+ T cells that recognize infected cells and kill them. This response does not prevent infection, but it can limit the spread of the virus and reduce disease severity. Because internal proteins are often more conserved than surface antigens, T cell responses may recognize a broader range of strains.

Antigenic variability constrains antigen choice

If the chosen antigen is the same surface protein that undergoes antigenic drift, the vaccine will need frequent updates to match circulating strains. If the antigen is a conserved internal protein, the response may be broader but less able to prevent infection. The choice is therefore a trade-off between breadth and the ability to block entry.