Clinical immunology — Which immune cell type is especially depleted in advanced AIDS, explaining many opportunistic infections?

Introduction:
Acquired immunodeficiency syndrome (AIDS) results from chronic HIV infection and progressive immune collapse. This question asks which lymphocyte subset is characteristically reduced to low counts in advanced disease.

Given Data / Assumptions:

• Advanced HIV infection with opportunistic infections present.
• Typical laboratory monitoring (CD4 counts, viral load).
• Pathophysiology of HIV tropism.

Concept / Approach:
HIV targets CD4+ helper T cells, using CD4 as a receptor along with chemokine coreceptors. Progressive depletion of CD4+ cells impairs both humoral and cellular arms by removing essential helper function for B cells and CD8+ T-cell responses.

Step-by-Step Solution:

Verification / Alternative check:
Guidelines use CD4 thresholds (e.g., 200 cells/µL) to trigger prophylaxis for opportunistic infections, illustrating the central role of CD4 depletion.

Why Other Options Are Wrong:

• CD8+ killer T cells: may be relatively preserved early; the hallmark is CD4 loss.
• B cells: dysregulated but not the defining depleted subset.
• None of these: incorrect; CD4+ loss is well established.
• NK cells: function may be altered, but profound CD4 depletion defines AIDS.

Common Pitfalls:
Assuming cytotoxicity alone determines susceptibility; without CD4 help, many immune functions fail despite CD8 presence.

Final Answer:
Helper T lymphocyte (CD4+).