Which phase describes the period in a primary response when antibody levels first become measurable?

• A. Lag phase
• B. Log phase
• C. Plateau
• D. Decline

Answer: (B)

In a primary antibody response, there’s a short period after antigen exposure when antibodies aren’t yet detectable—the lag phase. Once B cells become activated and start producing antibodies, the serum level begins to rise rapidly. This initial detectable rise corresponds to the exponential growth, or log, phase, where antibody concentration increases quickly and becomes measurable. The plateau follows as production and clearance balance out, and decline occurs as antibody levels fall. So the moment antibodies first become measurable is at the start of the log phase, when the response shifts from undetectable to rising levels.