A Level Biology Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

The correct answer is the identification of new antigens on the pathogen's surface. When a pathogen undergoes mutations, particularly in its proteins, it can create new antigens that the immune system has not previously encountered. The body recognizes these new antigens using its adaptive immune response, specifically by deploying B cells and T cells that can bind to and respond to these foreign substances. This process involves the activation of immune cells and the production of specific antibodies tailored to the new antigens, facilitating an effective immune response against the mutated strain.

While the body has mechanisms to respond to pathogens it has encountered before, the presence of new antigens means that the immune response may need to adapt. Relying solely on memory cells from previous infections or using the same response as for the original pathogen may not be sufficient if the mutations significantly alter the structure of the pathogen's antigens. Therefore, the recognition of new antigens is crucial for the immune system to mount an effective defense against mutated strains of pathogens.