🧬 Adaptive Immunity: Lymphocyte Receptors & Development

• Every lymphoid cell can be identified by the specific surface molecules it carries.
• A mature, never-yet-activated (naïve) B cell displays two antibody classes on its surface at once — IgM and IgD.
• A mature, naïve T cell displays a single receptor type — the T-cell receptor (TCR).
• Both receptor families are built from genes belonging to the same ancient superfamily of immune-recognition molecules.
• Random DNA-level shuffling during development produces millions of distinct versions of each receptor.

• Basic unit: a 4-chain glycoprotein built from 2 identical halves.
  • Each half = 1 heavy chain + 1 light chain.
  • Halves are joined by disulfide bonds into a Y-shaped molecule.
  • A flexible hinge region between the halves allows the two arms to move independently.
  • A tail on each heavy chain anchors the whole structure through the membrane.
• Antigen-binding site sits at the tip of each arm, where a heavy and light chain lie side by side.
  • Its unique 3-D shape = the idiotype — determines exactly which antigen it can grab.
  • Only one idiotype is made per B cell, even though it's displayed in multiple copies and as two isotypes (IgM + IgD).
• Can recognize essentially any type of molecule in its native, unprocessed form — protein, sugar, or lipid.
• Eventually gets remodeled into a secreted, freely circulating antibody once the cell becomes a plasma cell.

• Built from 2 chains of similar length — alpha and beta — each anchored through the membrane by a short tail.
• Antigen-binding groove forms between the two chains at their outer tips — this groove is the TCR's idiotype.
• No hinge region exists, so the TCR is structurally rigid, unlike the flexible BCR.
• Only binds a short peptide fragment that is already sitting in the groove of an MHC molecule on another cell's surface — never a free-floating antigen.
• Always stays membrane-bound; unlike the BCR, it is never released as a soluble, circulating molecule.

• Neither the BCR nor the TCR can signal on its own — each partners with dedicated accessory chains that carry the "antigen has bound" message into the cell.
• B-cell signaling partners
  • A pair of invariant chains, Ig-alpha and Ig-beta, sit alongside the BCR.
  • A separate co-receptor complex — CD19, CD21, CD81 — amplifies the signal.
  • CD21 doubles as the cell-entry receptor exploited by Epstein-Barr virus.
  • CD81 doubles as the cell-entry receptor exploited by hepatitis C virus and by Plasmodium vivax.
• T-cell signaling partner
  • A single multi-chain complex called CD3 sits alongside the TCR and transmits its signal.
• End result of either pathway: a cascade of intracellular phosphorylation events that switches the lymphocyte on.

• Problem: the body needs millions of distinct receptor shapes, but the genome cannot afford to dedicate a separate gene to each one.
• Solution: a set of DNA-shuffling tricks performed only inside developing lymphocytes.

Step-by-step for the heavy (or TCR beta) chain

1. Germline DNA holds many separate copies of three types of gene segments: Variable (V), Diversity (D), and Joining (J).
2. A developing lymphocyte randomly picks one of each and splices them together → VDJ recombination.
3. This assembled VDJ segment is transcribed, spliced onto a constant-region gene, and translated into the finished chain.

Step-by-step for the light (or TCR alpha) chain

• Same idea, but only two segment types are shuffled — V and J (no D segment).

The enzymes that make this possible

• RAG1 and RAG2 — two proteins that form the core of the recombinase complex; they cut and paste the chosen gene segments together, alongside built-in DNA-repair machinery.
• Terminal deoxynucleotidyl transferase (TdT) — randomly inserts extra bases at the V–D and D–J junctions with no template strand (N-nucleotide addition), adding another layer of unpredictability.
  • Active during heavy/beta chain rearrangement in both B and T cells.
  • Stays active for every chain of the TCR, but switches off before the light chain of the BCR is rearranged.

Quality control and further diversity

• Many rearrangements produce a broken, non-functional chain — the cell gets a second attempt on the paired (homologous) chromosome.
• If both attempts fail, the cell is instructed to self-destruct (apoptosis).
• Once one chromosome succeeds, further rearrangement of the other copy is permanently shut off — allelic exclusion — guaranteeing exactly one receptor specificity per lymphocyte.
• Any finished heavy/beta chain can pair with any finished light/alpha chain — this combinatorial mixing multiplies the total number of possible receptors even further.

• The bone marrow is a primary lymphoid organ because it is where B cells complete their entire maturation program.
• Maturation stage can be read directly off which immunoglobulin chains a cell is making.

• After antigen encounter in the periphery, the mature B cell can become:
  • An activated / blast B cell → plasma cell (cytoplasmic Ig+, antibody factory).
  • A memory B cell (surface IgG+, IgA+, or IgE+ depending on class switch).
• Markers that track the process: RAG expression and TdT mark the earliest rearranging stages; MHC class II and the CD19/CD20/CD21/CD40 cluster are expressed from the pro-B stage onward.

Central Tolerance — Removing Self-Reactive B Cells

• Because V(D)J rearrangement is random, some resulting receptors will accidentally bind the body's own molecules.
• Bone marrow stromal and interdigitating cells screen for this before release to the periphery.
  • Clonal deletion — cells with too-high affinity for self are eliminated in the marrow.
  • Clonal anergy — self-reactive cells that escape deletion are instead permanently inactivated in the periphery; these anergic cells show unusually high surface IgD.
• Net effect: only B cells that are selectively unresponsive (tolerant) to self-antigens are allowed to leave the marrow.

• T-lineage progenitors leave the bone marrow while still immature and travel to the thymus, the second primary lymphoid organ.
• Thymus anatomy: a two-lobed organ above the heart with an outer cortex (packed with immature cells) and an inner medulla (where maturing cells arrive); both zones contain epithelial cells, dendritic cells, and macrophages that interact directly with developing thymocytes.

Stages by CD4/CD8 Expression

The Two-Step Selection Filter (in the cortex, using MHC)

• MHC is a set of highly variable genes on chromosome 6, also called human leukocyte antigen (HLA); expressed at high density on thymic stromal cells.
  • Class I products: HLA-A, HLA-B, HLA-C.
  • Class II products: HLA-DP, HLA-DQ, HLA-DR (plus HLA-DM, a chaperone that loads peptide but never itself sits on the surface).
• Step 1 — Positive selection: double-positive thymocytes whose TCR binds self-MHC with low affinity survive and continue maturing; a TCR with zero affinity for self-MHC fails to be positively selected and dies by neglect.
• Step 2 — Negative selection: thymocytes whose TCR binds self-MHC + self-peptide too strongly are eliminated by apoptosis, since such cells would drive autoimmunity.
• Outcome: 95–99% of all T-cell precursors entering the thymus die during this filtering — only appropriately tuned cells graduate.

Becoming Single Positive

• A double-positive thymocyte whose TCR recognizes class I MHC keeps CD8 and drops CD4 → destined to become a cytotoxic T lymphocyte (CTL).
• A double-positive thymocyte whose TCR recognizes class II MHC keeps CD4 and drops CD8 → destined to become a helper T cell (Th).

Regulatory T Cells — An Exception to Deletion

• A subset of self-reactive thymocytes is diverted into becoming regulatory T cells (Tregs) instead of being deleted.
• Identified by constitutive CD25 expression plus the transcription factor FoxP3.
• Secrete IL-10 and TGF-β to dampen inflammation.
• Leave the thymus and enforce tolerance out in the periphery by suppressing self-reactive helper T cells.

▶ Thymic selection & T-cell fate · @StepWiseUSMLE

• TdT positivity in a leukemic blast points to an early B- or T-lymphoblastic process — it marks cells still in the rearrangement stage.
• MHC class II presents to CD4+ helper cells; MHC class I presents to CD8+ cytotoxic cells — mixing these up is one of the most common exam errors.
• Allelic exclusion is the reason a single lymphocyte never expresses two different antigen specificities at once.
• Failure of central B-cell tolerance (clonal deletion/anergy) or central T-cell negative selection is a mechanistic route toward autoimmune disease.
• CD21 and CD81 being viral entry receptors is a favorite integrative question linking immunology to infectious disease.