Questions: Adaptive Immunity and Lymphocyte Diversity

Question 1

A person is vaccinated against influenza. Two years later they are exposed to the same strain. Which feature of adaptive immunity explains why they fight off the infection faster than the first time?

• A. Their innate immune system has been permanently strengthened by the vaccine
• B. The vaccine increased the diversity of V(D)J recombination products
• C. Clonal selection during the first exposure generated long-lived memory lymphocytes specific to influenza antigens
• D. The adaptive response activates more quickly on second exposure because it no longer needs to wait for innate immune signals

Answer: (C)

During the first exposure (or vaccination), B and T cells specific to influenza antigens are activated and clonally expanded. A subset of these differentiate into memory cells that persist for years. On re-exposure, these memory cells recognize the antigen quickly and mount a faster, stronger response than the naive cells did initially. This is immunological memory — the defining feature of adaptive immunity that makes vaccination effective.

Question 2

The adaptive immune response is faster than the innate immune response because it generates a more targeted, antigen-specific reaction.

• T. True
• F. False

Answer: False

The adaptive immune response is actually slower than the innate response, not faster. Innate immunity activates within minutes to hours because it uses pre-formed, broadly reactive components (pattern recognition receptors, complement, natural killer cells). The adaptive response requires days to over a week for naive lymphocytes to encounter their specific antigen, clonally expand, and differentiate into effector cells. The advantage of adaptive immunity is specificity and memory, not speed.

Question 3

Explain how V(D)J recombination enables the immune system to recognize virtually any antigen, including pathogens that have never been encountered before.

Model answer: V(D)J recombination randomly assembles the variable region of T cell receptors and B cell receptors from separate gene segments (V, D, and J). Because there are many variants of each segment and the joining is imprecise (adding or deleting random nucleotides at junctions), the combinatorial and junctional diversity generates an estimated 10^15 to 10^18 unique receptor sequences — enough to bind virtually any molecular shape, including novel antigens.

Each lymphocyte expresses a unique receptor generated during its development. The key innovation is that the diversity is created before antigen exposure — the immune system pre-generates an enormous library of specificities. When an antigen enters the body, it selects (from this pre-existing library) the lymphocytes whose receptors happen to fit, which then clonally expand. This is Burnet's clonal selection theory: antigen selects; it does not instruct.