Final answer:
Antigenic variation, immunosuppressive molecules, and inhibiting phagocytosis are biological properties of pathogens that can block the successful design of an effective vaccine.
Step-by-step explanation:
One biological property of a pathogen that could block the successful design of an effective vaccine is antigenic variation. This is the alteration of surface proteins by the pathogen, so that it is no longer recognized by the host's immune system. For example, the bacterium Borrelia burgdorferi, which causes Lyme disease, undergoes antigenic variation of a surface lipoprotein called VlsE. This variation makes it difficult for antibodies against previous VlsE sequences to be effective.
Another mechanism that can hinder vaccine design is the production of immunosuppressive molecules by pathogens. Viruses, in particular, are adept at evading the immune response by suppressing the host immune function in ways more subtle than the destruction caused by HIV. This can impair the body's ability to generate sufficient immune responses to infections or tumors.
Finally, some pathogens can hinder the immune system by producing capsules, toxins, or other factors that inhibit phagocytes from engulfing them or displaying antigens to the adaptive immune system. For example, Streptococcus pneumoniae, the bacterium that causes pneumonia and meningitis, surrounds itself with a capsule that inhibits phagocytes from engulfing it.