Final answer:
The IL-2 receptor alpha chain acts as an affinity modulator for the IL-2 receptor complex without signaling capacity, whereas the beta and gamma chains are important for signaling.
Step-by-step explanation:
The correct answer to which cytokine receptor subunit acts as an affinity modulator but possesses no signaling capacity is the IL-2 receptor alpha chain. This receptor chain is known for increasing the affinity of the interleukin-2 (IL-2) receptor complex for its ligand IL-2 without directly transducing a signal when the ligand is bound. In contrast, the IL-2 receptor beta and gamma chains play critical roles in signal transduction for cell proliferation and differentiation. These receptors are integral parts of the adaptive immune response, with IL-2 signaling being crucial for T cell function.
For the T cell receptor, the alpha and beta chains each have constant and variable regions which recognize antigens, but this mechanism is different from the cytokine receptor discussed in the question. The function of the cytokine receptors is essential in understanding how the immune system responds to pathogens and regulates the immune response.