Which of the following is NOT a characteristic of an effective immunogen?

• A. Internal complexity
• B. Large molecular weight
• C. The presence of numerous epitopes
• D. Found on host cells

Answer: (D)

An immunogen is something capable of triggering an immune response, which depends on being foreign, sufficiently large, and structurally complex with multiple recognizable sites (epitopes). Internal complexity and large molecular weight both increase the likelihood that the substance will be processed and presented by antigen-presenting cells, creating multiple epitopes for B and T cells to recognize. Having numerous epitopes further raises the chance of a robust and diverse immune response.

What breaks the pattern is something found on host cells. Self components are usually tolerated by the immune system due to tolerance mechanisms, so they are not effective immunogens. In other words, being a self molecule found on host cells makes an immune response unlikely, not likely to be immunogenic.

So the trait that is not typical of an effective immunogen is being found on host cells.