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A given B cell expresses _____ of the light-chain loci.

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Final answer:

A given B cell expresses one of the light-chain loci. The genetic rearrangement process with over 106 possible combinations ensures each B cell has a unique variable chain that can bind specific antigens, underpinning the vast antibody diversity in the human immune system.

Step-by-step explanation:

A given B cell expresses one of the light-chain loci. During the maturation of germ-line B cells, the variable region of the light chain gene, which has 40 variable (V) and five joining (J) segments, undergoes genetic rearrangement through a process involving an enzyme called DNA recombinase. This enzyme randomly excises most of these segments and splices one V segment to one J segment. After RNA processing, which splices out all but one V and J segment, a unique VJ combination is produced for each differentiated B cell. This genetic rearrangement process allows for the creation of over 106 possible VJ combinations, resulting in each B cell having a unique variable chain. Moreover, antibody diversity is enhanced by the proportion of kappa and lambda light chains in immunoglobulin molecules, with humans having about a 2:1 ratio of kappa to lambda.

The constant domain of the light chains, which does not bind antibodies, is the same across all antibodies. Each mature naïve B cell, which has potentially 100,000 BCRs on its membrane, displays identical epitope-binding specificity tailored to bind specific pathogen epitopes. When a B cell matures to a plasma cell, it secretes antibodies that share structural similarities with the BCR's extracellular component.

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