MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient is experiencing chronic fatigue, weight loss, and frequent infections. Blood tests reveal significantly reduced numbers of B lymphocytes and T lymphocytes, but normal red blood cell and platelet counts. This condition is most likely due to a defect in which component of the immune system?
- ASpleen
- BBone marrow
- CThymus
- DLymph nodes
Show answer & explanationAnswer & explanation
Correct answer: B. Bone marrow
Both B lymphocytes and T lymphocytes originate from hematopoietic stem cells in the bone marrow. A defect in the bone marrow's ability to produce these precursor cells would lead to a reduction in both types of lymphocytes, while other blood cell lines might remain unaffected.
Why the other options are wrong
- A. The spleen filters blood and stores lymphocytes, but it's not the primary site of their production.
- C. The thymus is where T lymphocytes mature, but it's not where B lymphocytes mature or where either originates.
- D. Lymph nodes are sites where lymphocytes gather and interact, but not where they are initially produced in large numbers.
Hematopoiesis
Hematopoiesis is the process of blood cell formation, occurring primarily in the bone marrow in adults. All blood cells, including red blood cells, white blood cells (lymphocytes, neutrophils, etc.), and platelets, originate from hematopoietic stem cells.
- Occurs in bone marrow in adults.
- Produces all types of blood cells.
- Involves hematopoietic stem cells.
- Essential for immune function and oxygen transport.
Memory trick: Bone Marrow: The 'Bone' factory for all 'Blood' cells.