Immunology: Immune Recognition, Tolerance, and Regulation

Introduction to Immunology

Immunology explores how the immune system distinguishes self from non-self, generates vast antigen receptor diversity, mounts effective responses against pathogens, maintains tolerance to self-antigens, and forms long-lasting memory. These processes depend on precisely orchestrated networks of cells, cytokines, and molecular signals. When dysregulated, they lead to allergy, autoimmunity, immunodeficiency, or failure to control cancer. Modern immunotherapy has revolutionised oncology by harnessing these mechanisms.

The immune system operates on multiple levels: physical barriers (skin, mucosas), innate immunity acting within minutes using germline-encoded pattern recognition, and adaptive immunity developing over days but providing exquisite specificity and memory. Innate immunity detects conserved microbial patterns through Toll-like receptors, NOD-like receptors, and cGAS-STING pathways. Adaptive immunity uses randomly generated antigen receptors on lymphocytes, with clonal selection amplifying cells recognising specific antigens to high numbers.

Lymphocyte Activation and Receptor Diversity

T Cell Receptor Signalling

T cells recognise peptide fragments presented by MHC molecules. TCR engagement triggers ZAP-70 phosphorylation, LAT scaffold assembly, PLCgamma activation generating IP3 and diacylglycerol, calcineurin-NFAT nuclear translocation, and PKC-theta activating NF-kB. Full T cell activation requires co-stimulatory signals (CD28-B7 interaction) alongside TCR engagement. Signal one (TCR) without signal two (co-stimulation) induces anergy—a peripheral tolerance mechanism preventing responses to innocuous antigens presented without inflammation. CTLA-4 competes with CD28 for B7 and delivers inhibitory signals.

B Cell Affinity Maturation

B cells recognise antigens through their surface immunoglobulin B cell receptor. In germinal centres of lymphoid organs, activation-induced cytidine deaminase (AID) introduces somatic hypermutations into antibody variable region genes. B cells with highest-affinity mutations receive survival signals from follicular helper T cells; low-affinity cells die. This iterative process—affinity maturation—generates the high-affinity antibodies that mediate durable protective immunity. AID also mediates class switch recombination, changing antibody isotype from IgM to IgG, IgA, or IgE, altering effector function while maintaining antigen specificity.

Tolerance and Regulatory Mechanisms

Central and Peripheral Tolerance

Central tolerance eliminates autoreactive lymphocytes during development. In the thymus, T cells with high affinity for self-peptide/MHC complexes undergo negative selection. AIRE (autoimmune regulator) drives expression of peripheral tissue antigens in thymic medullary epithelium, enabling deletion of T cells that would attack these tissues. AIRE mutations cause autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). In the bone marrow, autoreactive B cells undergo receptor editing—alternative light chain rearrangement to change specificity—or deletion. Peripheral tolerance mechanisms including anergy, regulatory T cells, and immune privilege sites supplement central mechanisms.

Regulatory T Cells

Foxp3+ regulatory T cells (Tregs) maintain immune homeostasis through multiple mechanisms: IL-10 and TGF-beta secretion suppressing effector T cells, CTLA-4 competing for B7 co-stimulatory ligands, and IL-2 consumption depriving effector cells of survival signals. Loss-of-function FOXP3 mutations cause IPEX syndrome—severe multi-organ autoimmunity affecting males, demonstrating Tregs' essential role in preventing autoimmunity. Therapeutic Treg manipulation is pursued for autoimmune disease (increasing Tregs) and anti-tumour immunity (decreasing Tregs in the tumour microenvironment).

Immunological Memory and Applications

Immunological memory confers faster, stronger responses upon re-exposure to previously encountered antigens—the basis of vaccination. Memory B cells and long-lived plasma cells persist for decades, maintaining circulating antibody levels. Memory T cells circulate as central memory cells (lymph node-homing, long-lived) and effector memory cells (tissue-patrolling, immediate effector function). Tissue-resident memory T cells in lung, gut, and skin provide immediate local responses at barrier surfaces. Understanding memory formation principles guides vaccine design targeting optimal memory subsets and durability.

Examples and Applications

Example 1: PD-1/PD-L1 Checkpoint Immunotherapy

PD-1 on T cells engaged by PD-L1 on tumour cells suppresses T cell effector functions, enabling immune evasion. Anti-PD-1 antibodies (pembrolizumab, nivolumab) block this interaction, restoring anti-tumour T cell activity. FDA approval in melanoma (2014) opened the checkpoint immunotherapy era transforming oncology—durable responses in cancers previously uniformly fatal. Biomarkers predicting response include PD-L1 expression level, tumour mutational burden (more neoantigens), and microsatellite instability.

Example 2: CAR-T Cell Therapy

Chimeric antigen receptor T cells combine antibody-derived antigen binding domains with T cell signalling domains. Patient T cells are extracted, gene-modified ex vivo, expanded, then infused to attack cancer. CD19-targeted CAR-T cells achieve complete remission in 70-90% of relapsed/refractory B cell leukaemia patients—remarkable in previously terminal disease. Cytokine release syndrome and neurotoxicity are significant toxicities. Next-generation CAR-T cells target multiple antigens simultaneously, include built-in safety switches, and use allogeneic ('off-the-shelf') donor cells.

Example 3: mRNA COVID-19 Vaccines

mRNA vaccines encoding SARS-CoV-2 spike protein delivered in lipid nanoparticles demonstrated 95% efficacy against symptomatic COVID-19. The mRNA instructs host cells to produce spike protein, generating both antibody and T cell responses. Speed of development (under a year from sequence to approval) reflected decades of mRNA and lipid nanoparticle technology development. The platform is now being applied to influenza, HIV, cancer neoantigen vaccines, and other infectious diseases.

Example 4: Allergy Immunotherapy

Allergic responses involve IgE-mediated mast cell degranulation releasing histamine. Subcutaneous allergen immunotherapy gradually increases allergen dose over months to years, shifting responses from Th2-polarised (IgE, mast cell) toward Treg and Th1 (IgG4), reducing clinical allergy. Sublingual immunotherapy offers an at-home alternative. Both approaches achieve long-lasting tolerance—distinct from pharmacotherapy which only suppresses symptoms. Understanding tolerance mechanisms enabled rational immunotherapy design.

Example 5: Stem Cell Transplantation Immunology

Allogeneic haematopoietic stem cell transplantation treats haematological malignancies through graft-versus-leukaemia (GVL) effect—donor T cells attacking residual leukaemic cells. However, donor T cells also attack host tissues, causing graft-versus-host disease (GVHD). Separating GVL from GVHD is a central challenge; regulatory T cells and targeted depletion of alloreactive T cells aim to preserve GVL while eliminating GVHD. Matching by HLA type reduces but does not eliminate GVHD risk.

Example 6: Innate Lymphoid Cells in Asthma

Type 2 innate lymphoid cells (ILC2s) in the lung rapidly respond to epithelial-derived cytokines (TSLP, IL-33, IL-25) released by allergens or viruses, secreting IL-4, IL-5, and IL-13 before adaptive T cells are activated. IL-5 promotes eosinophil survival and trafficking to airways; IL-13 drives mucus production and airway hyperresponsiveness. Anti-IL-5 biologics (mepolizumab) and anti-IL-4Ralpha (dupilumab) treating severe asthma validate these pathways as therapeutic targets, demonstrating how basic ILC biology translates to clinical benefit.

Example 7: Systemic Lupus Erythematosus

SLE is a systemic autoimmune disease characterised by autoantibodies against nuclear antigens (dsDNA, histones). Defective clearance of apoptotic cells releases nuclear material activating innate (cGAS-STING, TLR7/9) and adaptive immune responses. Type I interferon elevation drives inflammation; genetic risk loci include complement components and immune regulatory genes. Belimumab (anti-BLyS) reduces B cell survival and was the first new lupus therapy in 50 years; anifrolumab (anti-interferon receptor) further validated type I IFN pathway as a therapeutic target.

Example 8: Neonatal Fc Receptor and Passive Immunity

Neonatal Fc receptor (FcRn) mediates transplacental IgG transfer from mother to foetus and extends IgG half-life by recycling antibodies from degradation. Maternal IgG protects neonates before their own immune system matures. FcRn is exploited in antibody engineering to extend therapeutic antibody half-life (Fc mutations increasing FcRn binding at pH 6 improve recycling). FcRn blockade with efgartigimod rapidly depletes all IgG—including pathogenic autoantibodies in diseases like myasthenia gravis and IgG4 nephropathy—providing a novel therapeutic mechanism.

Try it live

Everything above runs in your browser — open Thymic Selection Simulator: How T Cells Learn Self from Non-Self and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.