Active humoral immunity is characterized by which of the following?

• A. Passive Transfer of Antibodies from Another Individual
• B. Destruction by Cytotoxic T Cells
• C. Complement-Mediated Lysis Only
• D. Occurs When B Cells Encounter Antigens and Produce Antibodies

Answer: (D)

Active humoral immunity happens when B cells directly recognize a foreign antigen and then produce antibodies. After B cells bind the antigen and receive help from helper T cells, they proliferate and differentiate into plasma cells that secrete specific antibodies, as well as memory B cells for quicker recall later. These antibodies circulate to neutralize pathogens, tag them for destruction, and can activate complement—if needed—but the defining feature is the body's own production of antibodies in response to the antigen. The other options describe different scenarios: receiving antibodies from another person is passive immunity, cytotoxic T cells mediate cell-mediated immunity, and while antibodies can engage complement, active humoral immunity centers on B cells generating antibodies themselves.