A patient receives a vaccine and, days later, their plasma cells begin producing large quantities of antibodies that neutralize the antigen. Which type of immune defense is demonstrated?

Rationale: A. Innate immunity is incorrect because it provides immediate, non-specific defense against pathogens and does not involve antibody production. Examples include physical barriers like skin, phagocytic cells, and inflammatory responses. B. Autoimmune response is incorrect because this occurs when the immune system attacks the body’s own cells, rather than responding to a foreign antigen introduced by a vaccine. C. Humoral immunity is correct because it involves B lymphocytes (B cells) and plasma cells producing antibodies that circulate in the blood and lymph to neutralize specific antigens. Vaccines stimulate this adaptive immune response, creating immunologic memory for future protection. D. Cell-mediated immunity is incorrect because this arm of the immune system involves T lymphocytes that directly attack infected or abnormal cells, rather than producing antibodies. It is important for fighting viruses, intracellular pathogens, and tumor cells.