Active humoral immunity is characterized by which of the following?

• A. Natural Passive Transfer of Antibodies
• B. Passively Acquired Antibodies Without B Cell Activation
• C. Production of Memory B Cells After Antigen Exposure
• D. B Cells Encountering Antigens and Producing Antibodies

Answer: (D)

Active humoral immunity comes from B cells that meet a specific antigen, become activated, and differentiate into plasma cells that secrete antibodies. This antibody production by the activated B cells is the direct, functional outcome of the response and provides the circulating antibodies that neutralize pathogens. Memory B cells are also formed during this process, setting up faster responses if the same antigen is encountered again. The key point is the B cell activation leading to antibody production by plasma cells, which is what defines the active humoral response. The other descriptions refer to passive immunity—where antibodies are present without the B cells being activated—or to aspects that occur as a result of activation but do not capture the essential act of antibody production by B cells.