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An immune-cell lineage associated with antibody responses. Lineage-specific proliferation and clonal expansion should be distinguished from changes in a mixed sample.

A B cell displays an antigen-binding receptor and can differentiate into an antibody-secreting plasma cell or a memory biological population. Activation selects and expands particular binding specificities, allowing a rare lineage to contribute substantially to an immune response. ^[ref]

D. T. Fearon authored a 1980 study identifying a membrane glycoprotein as the receptor for the complement fragment C3b on human red blood cells, B cells, monocytes and polymorphonuclear leukocytes. The molecule, called gp205 in the paper, is associated with the receptor now called complement receptor 1. The work connected a purified protein's biochemical activity with a receptor found on several classes of blood cells.^[ref]

The route to identification is instructive. The glycoprotein was investigated for its ability to inhibit a C3 convertase and to assist enzymatic inactivation of C3b. Those activities suggested a C3b-binding function, which the receptor experiments tested. Thus a cell membrane receptor need not merely initiate a response: recognition of a complement component can also help regulate the amplification system to which that component belongs.^[ref]

A region of an activated secondary lymphoid follicle in which B cells proliferate and undergo selection.

A lymphocyte is an immune-cell class that includes B cells, T cells and natural killer cells. B cells and T cells generate antigen-specific biological populations central to adaptive immunity; natural killer cells use a different balance of activating and inhibitory receptors. After activation, selected lymphocytes can proliferate and differentiate, allowing a rare precursor biological population to generate a substantial response. The broad class therefore includes cells with distinct recognition and effector functions. ^[ref]

Yes: a microRNA can suppress tumour formation or progression by restraining an oncogenic programme, but that role must be demonstrated for a defined microRNA, tissue and genetic context. Reduced expression in a tumour or growth inhibition after massive microRNA mimic transfection is suggestive rather than sufficient evidence of an endogenous tumour-suppressor function.

microRNAs are short regulatory RNAs whose guide strands associate with Argonaute. Recognition through sequence complementarity, often involving a microRNA seed region and a target's 3′ untranslated region, can recruit translational repression and RNA decay machinery. Canonical precursor processing involves Drosha and Dicer, although not every microRNA follows every canonical step. A microRNA can affect many transcripts, and a transcript can receive input from several microRNAs. Therefore modest coordinated repression of several growth or survival factors can have a substantial phenotype even without complete elimination of one target. Suppression is a property of the regulatory network, not of an RNA being universally “anticancer”.

The let-7 family supplies a mechanistic example. Experiments connecting let-7-responsive sequences in Ras transcripts with regulation of Ras expression linked a developmental regulatory RNA to a conserved growth pathway.^[ref] Reduced let-7 in lung cancer is compatible with loss of this restraint, but the inverse expression relationship alone would not show causation. Functional experiments restoring let-7 in relevant lung-cancer systems suppressed tumour development, providing a stronger test of tumour-suppressive capacity.^[ref] Expression should still be compared with a physiological range, and one must distinguish preventing tumour initiation from treating an established tumour.

Genetic loss gives a complementary direction of evidence. The miR-15a and miR-16-1 cluster lies within a recurrently deleted region associated with chronic lymphocytic leukaemia. Deletion of the corresponding mouse regulatory region or the microRNA cluster increased B-cell proliferative abnormalities and produced clonal lymphoproliferative disease in a subset of animals.^[ref] Comparing the smaller cluster deletion with a larger deletion matters because neighbouring genes can contribute to a human copy-number association. Disease after specific microRNA loss supports endogenous restraint more directly than an expression signature does; incomplete penetrance and latency imply cooperation and context rather than an automatic one-step conversion of every b cell.

The p53–miR-34 connection illustrates both strength and limitation of a proposed pathway. p53 activation can induce miR-34-family transcription, and ectopic miR-34 family can restrain proliferation or promote death in suitable biological cells, connecting a transcriptional tumour suppressor with post-transcriptional repression.^[ref] Yet mice lacking all three miR-34 family members retained substantial p53-dependent arrest and apoptosis and did not show the predicted general increase in tumour susceptibility in the tested settings.^[ref] This does not prove that miR-34 family cannot suppress a tumour in any setting. It shows that a gain-of-function phenotype and placement downstream of p53 do not establish that the microRNA family is an indispensable general mediator of p53 tumour suppression.

A convincing analysis combines several evidence types. Measure mature guide RNA in defined malignant and normal biological cell populations, distinguishing it from precursor abundance and changes in stromal composition. Determine whether deletion, promoter silencing or altered processing explains loss. Reduce or delete the endogenous microRNA and test initiation or progression in a suitable model, with restoration from a controlled expression system. Compare related family members to expose redundancy. Use seed-mutant controls and multiple reagents to distinguish sequence-specific regulation from delivery toxicity or innate immune activation; deleting an entire processing enzyme is not a specific test of one microRNA because it affects many regulatory RNAs.

To identify consequential targets, combine binding evidence, early RNA/protein responses and target-site tests. A 3′-region reporter whose seed-complementary site is mutated supports direct sequence-dependent regulation, but the decisive endogenous test is to edit the site or express a suitably controlled microRNA-resistant target and ask whether it rescues the phenotype. Failure of one target to rescue can reflect regulation of several cooperating targets rather than absence of specificity. Conversely, rescuing only bulk proliferation does not establish which target controls invasion or metastasis. Measure relevant tissue phenotypes and target protein levels, not merely a pathway enrichment score from predicted targets.

Tumour-suppressive activity also does not guarantee an effective therapy. Delivery must reach the relevant biological cells and release the RNA into the correct compartment without excessive exposure of normal tissues. A clinical miR-34a microRNA mimic study encountered serious immune-mediated toxicity, illustrating that an endogenous regulatory rationale does not establish safe systemic replacement.^[ref] Target abundance, isoform-specific 3′ regions and competing pathways can change the effect of the same microRNA between tumours. microRNAs can be genuine tumour suppressors; the strongest evidence is specific endogenous loss, controlled restoration and mechanistic target tests, interpreted alongside genetic redundancy and tissue context.

A lymphatic gland is a lymph node; a Lieberkühnian gland is an intestinal crypt; a Brunner gland is a predominantly mucous duodenal gland. The first is a lymphoid organ with traversing lymphatic channels; the other two are epithelial glands opening towards the intestinal lumen. Similar historical terminology must not erase that difference.

A lymph node has a fibrous connective tissue capsule sending trabeculae into the organ and an internal reticular framework supporting mobile cells. Its outer cortex contains lymphoid follicles, whose appearance changes with immune activity; activated secondary follicles have paler germinal centres surrounded by more densely packed lymphocytes. The deeper cortex or paracortex lies between follicular cortex and medulla. The medulla contains branching cellular cords separated by medullary sinuses. A hilum provides the entry and exit of the main blood vessels and the exit of efferent lymphatics.

Afferent lymphatic vessels enter at the convex surface and deliver lymph to the subcapsular sinus. Flow then passes through cortical or trabecular channels into medullary sinuses and out through efferent vessels near the hilum. Macrophages and the reticular arrangement bring carried material into contact with the cellular apparatus. The node's blood circulation is distinct from that lymphatic route. In modern terms, follicles are predominantly B cell territories, the paracortex is rich in T cells, and medullary cords contain many antibody-producing plasma cells. Those cellular classifications go beyond an ordinary 1865 microscopic description.^[ref]

Thin stained sections show the overall architecture, while a silver stain highlights the supporting reticular fibres rather than every lymphocyte. The fibres include type III collagen; the thicker capsule has a different connective-tissue organization. One must not interpret the meshwork itself as an epithelial secretory tube, or assume a section missing the hilum proves that the node has no efferent vessel.^[ref]

The intestinal crypts are simple tubular epithelial invaginations opening between the intestinal villi in the small intestine. Their bases approach the muscularis mucosae, remaining within the mucosa and surrounded by lamina propria. Each gland has a lumen continuous with the intestinal surface and a lining supported by a basement membrane. It contains columnar epithelial cells at different stages of differentiation, goblet cells producing mucus, endocrine cells and, in small-intestinal crypt bases, conspicuous granular Paneth cells. A proliferative compartment supplies replacement cells that move towards the surface; this modern cell-renewal account explains why dividing cells are common in the crypts. Crypts also occur in the colon, whose mucosa lacks ordinary small-intestinal villi. One should therefore specify the region rather than make villi part of every crypt's definition.

A Brunner gland lies chiefly in the submucosa of the proximal duodenum, below the muscularis mucosae. It is a branched, compound tubular gland with pale mucus-rich secretory cells, basal nuclei and small lumina. Its ducts pass through the muscularis mucosae and commonly join overlying intestinal crypts before opening into the lumen. Its alkaline, mucus-rich secretion helps protect the mucosa from acidic gastric contents. The position below the muscularis mucosae distinguishes it particularly well from a Lieberkühn gland in the mucosa.^[ref]

The most useful comparison is consequently one of compartments: a reticular lymphoid organ containing sinuses, a mucosal epithelial tube, and a branched submucosal secretory system. Recognition of the capsule, epithelial lining, muscularis mucosae and duct connections establishes those distinctions more securely than a name alone.

A plasma cell is an antibody-secreting differentiated B cell. Extensive rough endoplasmic reticulum and a developed Golgi apparatus support the production and secretion of large amounts of antibody. Recognition of a particular antigen is inherited from the activated B cell lineage; cell differentiation changes the secretory programme rather than inventing specificity from scratch. Long-lived plasma cells can maintain antibody production after the initiating response has subsided. ^[ref]