Vaccines evoke an immune response because they contain:

Vaccines evoke an immune response primarily because they contain antigens that stimulate the body’s immune system to recognize and combat specific pathogens. Antigens are substances that the immune system identifies as foreign, prompting the production of antibodies and activating other immune responses.

When a vaccine is administered, it introduces these antigens—often in the form of weakened or inactive pathogens, or components of the pathogen such as proteins or sugars—allowing the immune system to develop a memory of these antigens. This memory enables the immune system to respond more rapidly and effectively upon future exposure to the actual pathogen, thereby providing protection against disease.

In contrast, the other options do not accurately represent how vaccines function. For example, while antibodies play a critical role in the immune response, they are not present in vaccines themselves; instead, they are produced as a reaction to the antigens in the vaccine. Similarly, B cells and T cells are components of the immune response that are activated by the presence of antigens, rather than being components of the vaccine itself. Thus, the presence of antigens is crucial to stimulating a protective immune response through vaccination.