Question 31 - Biological and Biochemical Foundations of Living Systems Practice Test for the MCAT

Passage 1: The most serious of the inherited immunodeficiencies is severe combined immunodeficiency disease (SCID). This disease is complex because it is caused by many different genetic defects. What groups them together is the fact that both the B cell and T cell arms of the adaptive immune response are affected.

Children with this disease usually die of opportunistic infections within their first year of life unless they receive a bone marrow transplant. Such a procedure had not yet been perfected for David Vetter, the “boy in the bubble,” who was treated for SCID by having to live almost his entire life in a sterile plastic cocoon for the 12 years before his death from infection in 1984. One of the features that make bone marrow transplants work as well as they do is the proliferative capability of hematopoietic stem cells of the bone marrow. Only a small amount of bone marrow from a healthy donor is given intravenously to the recipient. It finds its own way to the bone where it populates it, eventually reconstituting the patient’s immune system, which is usually destroyed beforehand by treatment with radiation or chemotherapeutic drugs.

Passage 2: Immune system T-cells recognize cell surface carbohydrates of bacterial and red blood cells. That these similarities have consequences for survival is indicated by the observation that individuals with blood type O are more susceptible to infection by Vibrio, the Gram-negative bacteria that causes cholera, and individuals with type A/B are more susceptible to infections from a broad range of E. coli variants, all of which are also Gram-negative.