GermlineSegmente

GermlineSegmente is a term used to describe DNA segments that reside in the germline genome and are inherited by offspring. In the context of adaptive immunity, these germline segments provide the donor material for somatic rearrangement that generates diverse antigen receptor genes in B and T lymphocytes. The most well-known germline segments are the variable (V), diversity (D), and joining (J) segments found at immunoglobulin and T-cell receptor loci.

Structure and organization

Germline segments are organized in loci that contain many V segments upstream of a smaller number of D and J segments, followed by constant region exons. Each segment is flanked by recombination signal sequences (RSS) that guide the rearrangement process. The exact number and layout of segments vary across species, reflecting evolutionary differences in immune strategies.

Mechanism and outcome

Somatic recombination reassembles selected germline V, D, and J segments in developing B and T cells. RAG1 and RAG2 enzymes mediate this recombination, and terminal deoxynucleotidyl transferase (TdT) adds random nucleotides at junctions to increase diversity. The rearranged variable regions pair with constant regions to form functional antigen receptors. Allelic exclusion helps ensure that a single receptor specificity is expressed per lymphocyte.

Functional and evolutionary considerations

Many germline segments are nonfunctional pseudogenes. The repertoire of functional segments, their arrangement, and the level of diversification differ among species, shaping immune system capabilities. GermlineSegmente thus underpin the potential diversity of antibodies and T-cell receptors available to recognize a vast array of antigens.