Which of the following events occurs first in the differentiation sequence of human B cells in the bone marrow?: (A) Cytoplasmic mu chains present in the B cell, (B) Immunoglobulin heavy chain rearrangement, (C) Immunoglobulin light chain rearrangement, (D) Surface IgD and IgM present on the B cell
Answer is B. The first event that occurs in the pre-B cell (progenitor) is the gene rearrangement of the heavy chain. The D gene and J gene recombination event occurs first, followed by V recombination with the D-J region. The cytoplasmic mu chains are the result of immunoglobulin heavy chain rearrangement, the production of the messenger RNA from this rearrangement, and the ribosomal synthesis of the protein mu chain. This is the second event that occurs in the B-cell maturational sequence. Light chain rearrangement occurs when recombination events occur with the V gene and J gene from the light chain germline. After this recombination has occurred, and the messenger RNA for this germ line has produced the light chain protein, the light chains and heavy chains form. The complete IgM molecule and IgD molecules are present on the surface of only the mature B cells. This event is the last event to occur during the differentiation and development of B cells in the bone marrow. Note: B cells perform two impoant functions: (1) they differentiate into plasma cells and produce antibodies, and (2) they can present antigen to helper T cells. Ref: Levinson W. (2012). Chapter 58. Cellular Basis of the Immune Response. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.