Biology · Immunity, Host Defence & Biological Interactions
Adaptive recognition, lymphocytes and antigen receptors
Suppose several lymphocyte lineages are present before a new antigen is encountered. Some receptors can bind the relevant structure and others cannot. A response can increase the descendants of selected lineages without the antigen teaching every cell how to build the same receptor.
Cells, Membranes & Organisation introduces selective boundaries; Gene Regulation & Cell Signalling explains receptors and communication. Homeostasis, Nutrition & Biological Organisation provides regulated compartments and physiological costs. The opening lessons define recognition, inflammatory signalling and cell movement before using them. Heredity, Variation & Evolution is a useful return path for clonal descendants and population change.
Back to the Biology mapBy the end, you can
Distinguish receptor diversity, recognition, activation and clonal expansion, and compare selected jawed-vertebrate and lamprey receptor systems.
Recognition depends on a repertoire that is already varied
An antigen is a structure recognised by the adaptive system; it need not belong to a dangerous organism. An epitope is a particular recognised part. Specificity means a pattern of molecular interaction, rather than a perfect one-to-one dictionary between one receptor and one whole microbial species. Cross-reactivity can allow one receptor to recognise related structures, and several different receptors can recognise parts of the same antigen.
In jawed vertebrates, developing B and T lymphocytes generate diverse antigen receptors through somatic rearrangement of inherited gene segments, with other processes contributing to diversity. Somatic means changes in the developing cell lineage; it does not mean that a response rewrites all cells or establishes a newly inherited sequence in the next generation. Early experimental evidence for rearrangement came from comparing immunoglobulin gene organisation in different mouse cell samples. That historical observation is a basis for the principle, not a complete account of present-day receptor assembly.
The constructed binding matrix assigns receptor R1 interaction with X and Y, R2 interaction with X, and R3 interaction with Z. These are symbolic recognition classes, with no sequences, chemical affinities or measured binding strengths. Recognising X selects more than one existing class, while R1’s overlap shows why specificity should not be equated with an absolute single-target rule.
Specificity can overlap without becoming indiscriminate
Read the diagram in words
R1 interacts with X and Y; R2 with X; R3 with Z. X is recognised by two classes. The matrix supplies no binding strength or cell-response measurement.
| Assigned receptor class | Target X | Target Y | Target Z |
|---|---|---|---|
| Receptor class R1 | Assigned interaction | Assigned interaction | No interaction assigned |
| Receptor class R2 | Assigned interaction | No interaction assigned | No interaction assigned |
| Receptor class R3 | No interaction assigned | No interaction assigned | Assigned interaction |
This recognition matrix does not specify full cell activation
Purely symbolic X/Y/Z targets and R1/R2/R3 receptor classes. No sequence, organism, affinity, activation threshold or practical receptor design is specified.
An interaction does not by itself specify every later response
Naïve lymphocytes have not yet undergone their full antigen-driven effector differentiation. In many selected responses, receptor engagement works with additional cellular signals and context. Antigen-presenting cells can connect innate sensing with T-cell activation, and helper-cell interactions contribute to many B-cell responses. Binding alone therefore cannot be assumed to produce a complete response in every cell.
Clonal expansion increases descendants of selected cells. Differentiation changes their activity, producing populations such as antibody-secreting cells and memory cells. The familiar B-cell receptor can interact with intact antigen; conventional T-cell receptors typically recognise a peptide displayed with an appropriate major histocompatibility complex molecule, abbreviated MHC. A free peptide and a peptide–MHC presentation are different recognition contexts. Some specialised T-cell populations use other arrangements, so the conventional comparison is not an exhaustive receptor catalogue.
Comparative immunity prevents the human arrangement becoming a universal definition. Lamprey research identified variable lymphocyte receptors assembled from a different kind of inherited sequence module. Jawless-vertebrate adaptive responses thus need not use the conventional jawed-vertebrate immunoglobulin receptor arrangement. The key comparison concerns diversity and lineage-specific recognition systems, rather than a ladder from ineffective to effective animals.
Selected founders contribute different ending proportions
Read the diagram in words
Two R1 founders each yield four descendants, giving eight. One R2 founder yields four. R3 stays one. The ending total is 13; R1 share is 8/13.
| Starting founders | Assigned divisions per founder | Descendants per founder |
|---|---|---|
| R1a | 2 | 4 |
| R1b | 2 | 4 |
| Starting founders | Assigned divisions per founder | Descendants per founder |
|---|---|---|
| R2a | 2 | 4 |
| Starting founders | Assigned divisions per founder | Descendants per founder |
|---|---|---|
| R3a | 0 | 1 |
| After assigned expansion | Ending cell count | Share of the ending population |
|---|---|---|
| Clone class R1 | 8 | 8/13 |
| Clone class R2 | 4 | 4/13 |
| Clone class R3 | 1 | 1/13 |
| Before expansion | 4 | Cells |
|---|---|---|
| After assigned expansion | 13 | Cells |
The assigned recognition pattern is held fixed
Original cell-count construction with an assigned activation context and no death, migration, mutation or receptor change. It is not a response-rate measurement.
Worked example
Expansion changes clone proportions without changing the assigned binding matrix
The starting account has two R1 cells, one R2 and one R3: four cells in total. Under the supplied activation context, the R1 and R2 founders each undergo two divisions. Each founder produces four descendants, so the ending counts are eight R1, four R2 and one unchanged R3, total 13.
R1 initially represented two of four cells, or one-half. After the assigned expansion, it represents eight of 13, about 61.5%. This denominator is the ending population, rather than the four founders. No X receptor was newly instructed into the R3 lineage in this case.
The expansion rule is deliberately simple. Real responses also include cell loss, selection and changing receptor properties during some B-cell responses. The construction holds each assigned binding pattern fixed and cannot determine whether a real antigen exposure would activate these classes.
Check your understanding
Does one receptor binding both X and Y contradict every useful meaning of specificity?
Show reasoning
No. Specificity describes a pattern of interaction and can include cross-reactivity. The assigned R1 pattern includes X and Y while excluding Z.
Does an antigen recognised by a B-cell receptor necessarily provide the complete conditions for every B-cell response?
Show reasoning
No. Activation and differentiation depend on the relevant cellular context and other signals in many responses. Recognition and full activation are separate events.
Why does the ending R1 fraction use 13 rather than four as its denominator?
Show reasoning
Thirteen is the ending total: eight R1 plus four R2 plus one R3. Four describes the starting founders, so it is not the denominator for the ending composition.
What to carry forward
A diverse repertoire, a recognition event and an expanded lineage are different parts of an adaptive response.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2e: animal immune responses (2018; selected sections in chapter 42)Mary Ann Clark, Matthew Douglas and Jung ChoiTextbook sectionsInnate responses and barriersAdaptive recognition and lymphocytesAntibody structure and functions
- Evidence for somatic rearrangement of immunoglobulin genes coding for variable and constant regions (PNAS 73(10):3628–3632, 1976; doi: 10.1073/pnas.73.10.3628)Nobumichi Hozumi and Susumu TonegawaPrimary research article
- Somatic diversification of variable lymphocyte receptors in the agnathan sea lamprey (Nature 430:174–180, 2004; doi: 10.1038/nature02740)Zeev Pancer, Chris T. Amemiya, Götz R. A. Ehrhardt, Jill Ceitlin, G. Larry Gartland and Max D. CooperPrimary research article