MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A clinical trial is testing a new drug designed to treat autoimmune disorders by suppressing the immune system. The drug is found to significantly reduce the proliferation and activity of T-lymphocytes. Which of the following immune responses would be most directly and severely impaired by this drug?
- AHumoral immunity mediated by antibodies
- BPhagocytosis by macrophages
- CComplement system activation
- DCell-mediated immunity against intracellular pathogens
Show answer & explanationAnswer & explanation
Correct answer: D. Cell-mediated immunity against intracellular pathogens
T-lymphocytes (T cells) are the central players in cell-mediated immunity. Cytotoxic T cells directly kill infected cells, and helper T cells activate other immune cells, including those involved in cell-mediated responses against intracellular pathogens. Suppressing T cells would therefore most severely impair this arm of the immune system.
Why the other options are wrong
- A. Humoral immunity is primarily mediated by B cells and antibodies, though helper T cells are crucial for B cell activation for most antigens.
- B. Phagocytosis is a function of innate immune cells like macrophages and neutrophils, not directly T-lymphocytes.
- C. The complement system is a part of innate immunity and can be activated by antibodies (classical pathway) or directly by pathogens (alternative/lectin pathways), not primarily T-cell dependent.
T-Lymphocyte Function
T-lymphocytes (T cells) are a type of white blood cell central to cell-mediated immunity, responsible for recognizing and eliminating infected cells, cancer cells, and coordinating immune responses.
- Helper T cells (CD4+) activate B cells, macrophages, and cytotoxic T cells.
- Cytotoxic T cells (CD8+) directly kill target cells presenting specific antigens.
- Regulatory T cells suppress immune responses to prevent autoimmunity.
- Key for defense against intracellular pathogens (viruses, some bacteria/fungi).
Memory trick: T-cells are 'CELL'ular 'COMMANDERS'.