Question

Given below are a few statements related to SCID. Pick out the statement that doesn’t relate to it.

• A. It is caused due to deficiency of the enzyme adenosine deaminase
• B. It results in the lack of both B - cells and T - cells
• C. It leads to the accumulation of homogentisic acid in the body
• D. The number of circulating thymocytes are low

Answer 1

The correct option is C It leads to the accumulation of homogentisic acid in the body

Adenosine deaminase (ADA) is an enzyme important for the immune system of our body. It is required for purine metabolism, where it converts deoxyadenosine, which is toxic to lymphocytes, into another harmless molecule called deoxyinosine. In the absence of ADA, the amount of deoxyadenosine increases and reduces the number of lymphocytes, ultimately affecting the immune system of the patient. This condition is referred to as SCID or Severe Combined ImmunoDeficiency.

B- cells and T- cells are important components of the immune system. Lymphocytes are agranular white blood cells (WBCs). The lymphocytes are developed from myeloid progenitor cells in the bone marrow. But the sites of maturation for B and T lymphocytes are different. T lymphocytes migrate to the thymus for maturation and B lymphocytes undergo maturation in bone marrow itself. In the thymus, the developing T cells are known as thymocytes. In SCID, the number of circulating thymocytes is reduced because of the inhibition of their maturation and cell death. Reduction in T- helper cells prevents the activation of B -cells, which affects the antibody-mediated immunity also.
Accumulation of homogentisic acid occurs in the body due to a genetic disorder called alkaptonuria. In this genetic disorder, the body is unable to produce an enzyme (phenylalanine hydroxylase) that is required for the breakdown of amino acid phenylalanine to tyrosine. This leads to the accumulation of homogentisic acid in the blood.