Immunology – identifying the negative phase In immunology, the phenomenon known as the “negative phase,” which represents a transient fall in antibody levels immediately after antigen exposure, is most prominently observed in which type of humoral immune response?

Introduction / Context:
The immune system exhibits different patterns of antibody response depending on whether the body is encountering an antigen for the first time (primary response) or during subsequent exposures (secondary response). A key phenomenon to understand is the “negative phase,” which refers to an initial dip in circulating antibody levels after antigen introduction.

Given Data / Assumptions:

• Antibody titers can fluctuate after antigen administration.
• Primary responses differ from secondary in timing and magnitude.
• Negative phase describes the short period of antibody decline before synthesis of new antibodies begins.

Concept / Approach:
In the primary immune response, existing natural antibodies (low-affinity IgM) may bind the introduced antigen, forming immune complexes. This temporarily reduces measurable free antibody levels, producing the negative phase. Later, B-cell activation and clonal expansion lead to rising IgM and IgG levels. In secondary responses, memory B cells rapidly generate antibodies, typically masking or eliminating any negative phase.

Step-by-Step Solution:
Step 1: Define negative phase = transient decline in antibody titer post-antigen exposure.Step 2: Link to mechanism: antigen binding clears pre-existing antibodies.Step 3: Identify context: occurs during first exposure, i.e., primary humoral response.Step 4: Conclude that the correct answer is “Primary humoral response.”

Verification / Alternative check:
Experimental antibody titer curves show a brief dip in antibody concentration before the classic lag, log, plateau, and decline phases of the primary response.

Why Other Options Are Wrong:

• Secondary response: rapid IgG rise, no clear negative phase.
• Both: not correct; it is characteristic of the primary response.
• None: incorrect, because the negative phase is well-documented in primary responses.
• CMI only: irrelevant; negative phase refers specifically to antibody titers.

Common Pitfalls:
Confusing the “lag phase” with the “negative phase.” The lag phase is the delay before antibody increase, while the negative phase specifically refers to the transient decline in circulating antibodies.

Final Answer:
Primary humoral response.