After exposure to an antigen, which period represents when no antibody can be detected yet?

• A. The lag phase
• B. The log phase
• C. The plateau phase
• D. The decline phase

Answer: (A)

After antigen exposure, there is a window before antibodies become detectable—the lag phase. This delay happens because B cells must first recognize the antigen, proliferate, differentiate into antibody-secreting plasma cells, and undergo any needed isotype switching. Until enough antibody is produced to reach detectable levels, serology tests will be negative. Once production ramps up, antibody levels rise rapidly in the log (exponential) phase, then level off during the plateau as production and clearance balance, and eventually decline as antibodies are cleared and production wanes. In short, the period with no detectable antibody yet is the lag phase.