![haptens have immunogenicity but not reactivity. haptens have immunogenicity but not reactivity.](https://image.slidesharecdn.com/antigen-160925143307/95/antigen-6-638.jpg)
The repertoire of a specific B-cell population can be studied separately by sorting the target population for example, sorting naive B cells allows the exploration of a naive BCR repertoire. For a naive B cell to become activated, it needs to bind an antigen and the activated B cell can then give rise to effector B cells and memory B cells. The BCR of a naive B cell is in its germline configuration without any somatic hypermutation. Each B cell has its unique BCR and its specificity is determined by heavy and light chain pairing, V(D)J recombination, combinatorial and junctional diversity and somatic hypermutation. While the TCR only exists as a membrane-bound form, the antigen-binding component of BCR can be produced also as water-soluble Ig molecules – antibodies – that exert effector functions in extracellular fluids (serum, mucosa). BCRs may further diversify by somatic hypermutation, which introduces point mutations in certain hotspots.
![haptens have immunogenicity but not reactivity. haptens have immunogenicity but not reactivity.](https://image.slideserve.com/338805/slide30-l.jpg)
Although the majority of rearranged Ig sequences contain only one D gene segment, VDDJ recombination has also been reported.
![haptens have immunogenicity but not reactivity. haptens have immunogenicity but not reactivity.](https://image.slidesharecdn.com/haptens-150727171343-lva1-app6891/95/haptens-5-638.jpg)
Note that D gene segments are only present in the heavy chain locus and are lacking in the kappa and lambda light chain loci. Immune repertoire diversity is mainly obtained by somatic gene rearrangements, so-called VDJ recombination, where one variable (V), one diversity (D) and one joining (J) gene segment combine in a supposedly stochastic fashion. The collection of BCRs (or TCRs) in an individual is called the BCR (or TCR) repertoire. To be able to recognise the vast plethora of existing antigens, an immense diversity of both BCRs and TCRs is required. Similarly, we refer to the TCR as the antigen-binding receptor without the CD3 co-receptor. Whereas functionality of a BCR is dependent on complex formation with the CD79a/b co-receptor, in this review, we refer to BCR as the antigen-binding, cell-surface anchored immunoglobulin without this co-receptor. Both types of adaptive immune cells recognise antigenic determinants by specific receptors – the B-cell receptor (BCR) and the T-cell receptor (TCR). Adaptive immune responses carried out by B and T cells are central in the body’s fight against pathogens, and they are also implicated in autoimmune diseases.