Final answer:
The principal protective immunity to influenza is a. antibodies which are produced as part of the humoral immune response. They help neutralize the virus upon a subsequent exposure.
Step-by-step explanation:
The principal protective immunity to influenza is antibodies. Antibodies are key players in the immune system's protection against viruses, including influenza. When an individual is exposed to an influenza virus, their immune system responds by producing antibodies specifically tailored to neutralize it. This response is part of what's known as the humoral immune system. The immunity provided by these antibodies allows the body to recognize and defend against the virus more effectively upon future exposures.
Additionally, other components of the immune system such as NK cells and cytotoxic T cells also play a role in protecting the body against the virus, particularly when it comes to destroying virally infected cells. However, it is the antibodies that primarily contribute to the protection against future exposures, by recognizing and targeting the virus before it can infect cells and replicate.
It is important to note that the effectiveness of antibodies can be compromised due to the antigenic properties of the influenza virus changing rapidly, potentially leading to new strains that the immune system doesn't recognize.