Which of the following events occur first in the differentiation sequence of human B cells in the bone marrow?
**Core Concept**
The differentiation sequence of human B cells in the bone marrow involves a series of specific events that lead to the maturation of immature B cells into mature B cells. This process is crucial for the development of the adaptive immune system and the production of antibodies.
**Why the Correct Answer is Right**
The correct answer is related to the rearrangement of immunoglobulin genes. The process begins with the rearrangement of the heavy chain genes, followed by the rearrangement of the light chain genes. This process is mediated by the enzyme terminal deoxynucleotidyl transferase (TdT), which introduces nucleotide diversity into the V(D)J recombination process. The correct answer is also related to the expression of the immunoglobulin heavy chain (IgH) gene, which is the first to be expressed during B cell development.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the rearrangement of the kappa light chain gene occurs later in the process, after the rearrangement of the heavy chain genes.
**Option B:** This option is incorrect because the expression of the immunoglobulin light chain (IgL) gene occurs after the expression of the IgH gene.
**Option C:** This option is incorrect because the activation of the B cell receptor (BCR) complex occurs later in the process, after the maturation of the B cell.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the rearrangement of immunoglobulin genes is a critical step in the development of B cells and is essential for the production of antibodies. Any defects in this process can lead to immunodeficiency disorders, such as severe combined immunodeficiency (SCID).
**Correct Answer: C. Expression of the immunoglobulin heavy chain (IgH) gene.**