Antigens, MHC, Mechanics Of Antigen Presentation

Antigen Recognition and Immunogenicity

In vertebrate immunology, the initiation of an adaptive immune response relies on the precise recognition of foreign compounds. This guide details the molecular and genetic mechanisms of antigens, the Major Histocompatibility Complex (MHC), and the pathways through which antigens are processed and presented to T lymphocytes.

1. Antigens vs. Immunogens

While often used interchangeably in casual discourse, there is a fundamental functional distinction between an antigen and an immunogen.

Core Definitions

Antigen: Any agent capable of binding specifically to a T-cell receptor (TCR) or an antibody molecule (membrane-bound or soluble). The ability of a compound to bind with these receptors is termed antigenicity.

Immunogen: Any agent capable of actively inducing a humoral (antibody-mediated) or cell-mediated immune response. This capacity is termed immunogenicity.

The Core Rule

All immunogens are antigens, but not all antigens are immunogens. Some small or simple compounds can bind to immune receptors (antigenic) but fail to stimulate an immune response on their own (non-immunogenic).

Molecular Requirements for Immunogenicity

For a substance to function as an effective immunogen, it must satisfy four key physiological and physicochemical criteria:

A. Foreignness (Non-Self Recognition)

The host’s adaptive immune system must recognize the compound as foreign. During lymphocyte development, cells reactive to self-molecules are eliminated or inactivated, creating self-tolerance. Self-antigens are non-immunogenic within the host but can act as highly potent immunogens when introduced into an individual of a different species (or even a genetically distinct individual of the same species).

B. Molecular Size

There is a direct correlation between the molecular mass of a compound and its immunogenicity:

Small Molecules (<1,000 Da): Generally completely non-immunogenic (e.g. penicillin, aspirin).
Intermediate Molecules (1,000–6,000 Da): May or may not exhibit immunogenicity (e.g. insulin, adrenocorticotropic hormone [ACTH]).
Large Molecules (>6,000 Da): Generally immunogenic (e.g. albumin, tetanus toxin).
Highly Active Immunogens: Typically possess a molecular mass of 100,000 Da or more.

C. Chemical Complexity and Composition

An immunogen must possess a minimum level of chemical complexity.

Homopolymers: Even with very high molecular weight, homopolymers of amino acids or sugars are extremely poor immunogens because they lack structural diversity. For instance, a homopolymer of poly-γ-D-glutamic acid (the capsular material of Bacillus anthracis) has a molecular weight exceeding 50,000 Da but remains non-immunogenic due to its lack of chemical complexity.
Heteropolymers: Virtually all proteins are highly immunogenic due to their complex, folded tertiary and quaternary structures composed of diverse amino acid residues.
Carbohydrates: Can be immunogenic if they exhibit complex polysaccharide branches or form part of glycoproteins.
Lipids and Nucleic Acids: Are generally poor immunogens on their own but become highly immunogenic when conjugated to protein carriers.

D. Dosage and Route of Administration

The outcome of antigen exposure depends on the amount administered and the route of entry. An insufficient dose fails to activate enough lymphocytes to trigger a response. Conversely, an excessively high dose can render the responding cells tolerant or unresponsive. A threshold dose is required to elicit an optimal immune response.

Subcutaneous (SC): Antigens injected beneath the skin generally elicit the strongest immune responses, due to rapid uptake and processing by Langerhans cells (highly active dendritic cells in the skin), which subsequently migrate to local lymph nodes.
Intradermal (ID): Injected directly into the skin; similarly effective at recruiting local dendritic cells.
Intramuscular (IM): Injected into muscle tissue; typically processed via local lymphatics.
Intravenous (IV): Carried directly to the spleen, where it can either induce systemic immune unresponsiveness (tolerance) or activate splenic lymphocytes, depending on dosage and physical form.
Oral (Gastrointestinal): Generally elicits local mucosal immunity, characterized by production of secretory immunoglobulin A (IgA) within the intestinal lamina propria.

2. Haptens and the Hapten-Carrier Effect

Core Definition

Haptens (from the Greek hapten, meaning “to grasp”) are small, chemically simple substances that are antigenic but non-immunogenic. In their native form, haptens fail to trigger an immune response because their low molecular weight and structural simplicity prevent them from cross-linking B-cell receptors or activating helper T cells.

When a hapten is covalently conjugated to a high molecular weight, immunogenic protein carrier (such as Bovine Serum Albumin [BSA] or Keyhole Limpet Hemocyanin [KLH]), the resulting hapten–carrier conjugate becomes highly immunogenic.

Immunological Outcome

Anti-hapten antibodies: specific to the hapten itself, and able to bind free, unconjugated hapten.
Anti-carrier antibodies: specific to the protein carrier.
Anti-junction antibodies: specific to the bridge or junction where the hapten is linked to the carrier.

Biophysics of Antigen-Antibody Interactions

3. Biophysics of Antigen-Antibody Interactions

Antigen-antibody (Ag-Ab) association is a highly specific, reversible process resembling the bimolecular interaction between an enzyme and its substrate.

Molecular Forces

The binding between an antigen and an antibody is mediated entirely by non-covalent interactions requiring close proximity (typically less than 1 × 10^-7 cm):

Hydrogen bonds
Electrostatic (ionic) interactions
Van der Waals forces
Hydrophobic interactions

Key Terminology

Terminology

Epitope (Antigenic Determinant): The smallest, restricted unit of an antigen that is directly bound by an antibody or recognized by a TCR.

Paratope: The antigen-binding site on an antibody molecule that directly accommodates the epitope.

Valency: The number of epitopes present on the surface of a single antigen molecule.

Monovalent: Possessing only one epitope copy.
Polyvalent / Multivalent: Possessing multiple epitope copies, allowing the antigen to bind several antibody molecules simultaneously.

Affinity vs. Avidity

These thermodynamic parameters describe the strength of the antigen-antibody complex:

Affinity measures the intrinsic binding strength of a single, non-covalent interaction between an individual epitope and a single paratope. It is mathematically defined by the association constant (K_a). Low-affinity antibodies bind weakly and dissociate rapidly, whereas high-affinity antibodies bind tightly and remain bound longer.

Avidity measures the overall, functional strength of attachment between a multivalent antigen and multivalent antibodies. Avidity is not a simple sum of individual affinities; rather, it represents a synergistic effect. When multiple epitopes on an antigen interact simultaneously with multiple binding sites on an antibody (such as pentameric IgM or dimeric IgA), the overall stability of the complex increases exponentially.

Key Takeaway

Even if the individual epitope-paratope affinity is low, high valency results in high avidity.

Cross-Reactivity

Although antigen-antibody interactions are highly specific, an antibody raised against one antigen can sometimes cross-react with an unrelated, non-immunising antigen. This cross-reaction occurs through two distinct molecular scenarios:

Two Molecular Scenarios

Shared Epitope: The two antigens contain a chemically identical epitope, though the rest of their structures differ.
Similar Epitope (Structural Resemblance): The second antigen possesses an epitope that is structurally and chemically similar to the immunising antigen’s epitope, permitting weaker, low-affinity binding.

Serology, Adjuvants, and the Major Histocompatibility Complex

4. Factors Influencing In Vitro Reactions (Serology)

Serology is the in vitro study of reactions involving serum constituents, primarily antibodies and complement. The detection and stability of these complexes depend on several physical and spatial parameters:

Antibody Affinity: High-affinity antibodies form more stable and detectable Ag-Ab complexes.
Antibody/Antigen Valency (Avidity): Multivalent interactions greatly increase the stability of the complexes.

Antigen-to-Antibody Ratio

The physical size and visual detection of complexes depend on the relative concentrations of antigen and antibody. Under the Zone of Equivalence, where the ratio of antigen to antibody is optimal, large, highly cross-linked lattices form. In the zones of antibody excess (Prozone) or antigen excess (Postzone), lattice formation is inhibited, reducing detection.

Physical Form of the Antigen

Particulate Antigens: Antigens bound to cell surfaces or insoluble particles undergo agglutination (visible clumping) when cross-linked by antibodies.
Soluble Antigens: Cross-linking leads to the formation of large, insoluble lattices that settle out of solution as a visible precipitation reaction.

Spatial Arrangement of Epitopes

Non-overlapping Epitopes: When epitopes are widely separated on an antigen, multiple antibody molecules can bind concurrently without interfering with one another.
Overlapping Epitopes: When epitopes lie in close proximity, the binding of one antibody physically blocks the binding of a second antibody through steric hindrance.
Allosteric Effects: The binding of an antibody to one epitope can induce a conformational change in the overall antigen structure. This change can either positively or negatively influence the binding of a second antibody at a distant site.

5. Adjuvants

Core Definition

An adjuvant (derived from the Latin adjuvare, meaning “to help”) is any substance that, when mixed with an immunogen and injected with it, non-specifically enhances the immune response against that immunogen. Adjuvants are crucial for boosting the immunogenicity of weak antigens (such as synthetic peptides or recombinant proteins).

Key Mechanisms of Action

Mechanisms

The Depot Effect: Adjuvants precipitate or sequester the antigen, slowing down its clearance and promoting its persistent, prolonged release at the injection site.
PRR Activation: Many adjuvants contain microbial components that directly stimulate Pattern Recognition Receptors (such as TLRs) on local APCs, driving the expression of co-stimulatory molecules (CD80/CD86) and pro-inflammatory cytokines.
Granuloma Formation: Adjuvants attract and activate macrophages and dendritic cells, creating a local inflammatory focus (granuloma) that optimises antigen uptake.

Common Adjuvants in Immunology

Alum (Aluminium Potassium Sulfate): The most widely used adjuvant in human vaccines. It acts primarily by precipitating the antigen to prolong its persistence and promoting a T_H2 (humoral) biased response.
Freund’s Incomplete Adjuvant: A water-in-oil emulsion containing the antigen in aqueous solution within light mineral oil, using an emulsifying agent like mannide monooleate. It acts primarily via the depot effect.
Freund’s Complete Adjuvant (FCA): Contains all the ingredients of the incomplete adjuvant plus heat-killed Mycobacterium tuberculosis or Mycobacterium butyricum. The mycobacterial components activate TLR2, TLR4, and TLR9, stimulating a highly robust, cell-mediated (T_H1) immune response.

Safety Note

Due to its severe inflammatory side effects, FCA is strictly restricted to laboratory animal research and is never used in human vaccines.

6. The Major Histocompatibility Complex (MHC)

Core Definition

The Major Histocompatibility Complex (MHC) is a tightly linked cluster of genes present in all jawed vertebrates. Its primary physiological role is to encode membrane-bound glycoproteins that present peptide antigens to T lymphocytes.

Genetic Characteristics of MHC

Chromosomal Localisation

Humans: Located on the short arm of Chromosome 6 and designated as the HLA (Human Leukocyte Antigen) complex.
Mice: Located on Chromosome 17 and designated as the H-2 (Histocompatibility-2) complex.

High Polymorphism

The MHC is the most polymorphic gene cluster known in vertebrates. Within a population, there are hundreds of alternative alleles for each locus (genetic polymorphism). This extreme diversity ensures that some individuals in a population can present peptides from virtually any mutating pathogen, preventing population-wide susceptibility.

Codominant Expression

Alleles of MHC genes are expressed codominantly. Both maternal and paternal copies of MHC genes are expressed simultaneously on the cell membrane of a single cell, maximizing the variety of peptides a single host can present.

Linkage and Haplotypes

Due to the close physical proximity of the MHC loci, the recombination rate within the complex is very low (<1%). Thus, the set of MHC alleles on a single chromosome is typically inherited as a single block or haplotype from each parent.

Inbred Mouse H-2 Haplotypes

In laboratory research, inbred mouse strains (produced by at least 20 consecutive generations of brother-sister mating) are homozygous at all loci and possess identical MHC haplotypes:

Prototype Strain	Key Congenic / Syngeneic Strains	H-2 Haplotype	H-2K Allele	IA Allele	IE Allele	S Allele	H-2D Allele
CBA	AKR, C3H, C57BR	k	k	k	k	k	k
DBA/2	BALB/c, SEA, YBR	d	d	d	d	d	d
C57BL/10 (B10)	C57BL/6, C57L	b	b	b	b	b	b
A	A/He, A/Sn	a	k	k	k	d	d

7. Structure and Classification of MHC Molecules

MHC genes are organized into three distinct structural classes:

Class I MHC Molecules

Gene Loci

Humans (HLA): Encoded by three classical loci: HLA-A, HLA-B, and HLA-C.
Mice (H-2): Encoded by classical loci: H-2K, H-2D, and H-2L.

Structural Architecture

MHC Class I molecules are membrane-bound heterodimers composed of two non-covalently associated polypeptide chains:

Polymorphic α (Heavy) Chain (~43 kDa): A Type I transmembrane glycoprotein encoded within the MHC gene cluster. It is organized into three extracellular domains (α₁, α₂, and α₃), a hydrophobic transmembrane segment, and a hydrophilic carboxyl-terminal cytoplasmic tail.
β₂-Microglobulin (β₂m, ~12 kDa): A non-polymorphic, invariant polypeptide encoded on a completely different chromosome (Chromosome 15 in humans). It does not span the membrane but associates non-covalently with the α₃ domain of the heavy chain.

Critical Note

Expression of Class I MHC on the cell membrane is strictly dependent on the assembly of the α chain with β₂m. In mutant cells lacking functional β₂m, Class I molecules fail to reach the cell surface.

Functional Domains

Peptide-Binding Cleft: Formed by the interaction of the α₁ and α₂ domains. These domains fold to create a flat floor of eight antiparallel β-pleated strands bordered by two long α-helices.
Features: The cleft is closed at both ends. Consequently, it can only accommodate short peptides, typically 8 to 10 amino acids long. The peptide is anchored by its amino and carboxyl termini to specific pocket residues in the cleft.
Co-Receptor Binding Site: The highly conserved α₃ domain contains the binding site for the CD8 glycoprotein present on cytotoxic T (T_C) cells.

Class II MHC Molecules

Gene Loci

Humans (HLA): Encoded by three classical loci: HLA-DP, HLA-DQ, and HLA-DR. Each locus contains distinct A and B genes encoding the α and β polypeptide chains, respectively (e.g. DPA1 and DPB1). The DR locus is particularly complex, containing a single DRA gene and up to nine DRB genes (DRB1 to DRB9).
Mice (H-2): Encoded by two classical loci: I-A and I-E.

Structural Architecture

MHC Class II molecules are membrane-bound heterodimers composed of two non-covalently associated transmembrane glycoproteins:

α Chain (~35 kDa): Composed of extracellular domains α₁ and α₂.
β Chain (~28 kDa): Composed of extracellular domains β₁ and β₂.

Both chains are polymorphic and encoded within the MHC gene cluster. Both chains span the plasma membrane and terminate in short cytoplasmic tails.

Functional Domains

Peptide-Binding Cleft: Formed by the interaction of the α₁ and β₁ domains.
Features: Unlike Class I, the Class II peptide-binding cleft is open at both ends. This allows it to accommodate longer, more variable peptides, typically 13 to 18 amino acids long, which can extend out of the cleft like a hot dog in a bun.
Co-Receptor Binding Site: The conserved β₂ domain contains the binding site for the CD4 glycoprotein present on helper T (T_H) cells.

Class III MHC Molecules

Unlike Class I and II, Class III genes do not encode antigen-presenting cell-surface receptors. Instead, they encode a diverse array of secreted proteins with immunological functions:

Complement components (C2, C4, and Factor B [Bf]).
Inflammatory cytokines (Tumour Necrosis Factor-alpha [TNF-α] and Lymphotoxin/TNF-β).
Heat Shock Proteins (Hsp70, which function as molecular chaperones).

MHC Expression, Antigen Processing, and Presentation Pathways

8. Cellular Expression and Comparison of MHC Classes

The cellular distribution of MHC Class I and Class II molecules reflects their distinct immunological roles:

Class I MHC Expression

Expressed constitutively on nearly all nucleated cells. The highest expression is found on lymphocytes, whereas lower levels are present on fibroblasts, muscle cells, liver hepatocytes, and neural cells. Sperm cells and neurons lack membrane-bound Class I MHC expression entirely.

Class II MHC Expression

Expressed constitutively only on professional Antigen-Presenting Cells (pAPCs):

Dendritic Cells: Considered the most powerful and efficient pAPCs, expressing exceptionally high baseline levels of Class II MHC and co-stimulatory molecules.
Macrophages: Upregulate Class II expression upon activation by phagocytic stimuli or cytokines.
B Lymphocytes: Constitutively express Class II MHC and use membrane Ig to internalise specific antigens for presentation to T_H cells.

Inducible Expression: Non-professional APCs (such as fibroblasts, thymic epithelial cells, thyroid epithelial cells, glial cells, and pancreatic beta cells) do not express Class II MHC under normal homeostatic conditions, but can be induced to express them when stimulated by the pro-inflammatory cytokine Interferon-gamma (IFN-γ).

Class I vs. Class II: Feature Comparison

Feature / Attribute	Class I MHC	Class II MHC
Structure	Heterodimer: α chain (43 kDa) + β₂m (12 kDa)	Heterodimer: α chain (35 kDa) + β chain (28 kDa)
Polymorphic Domains	α₁ and α₂	α₁ and β₁
Peptide-Binding Cleft	Closed at both ends	Open at both ends
Peptide Length Accommodated	Short (8–10 amino acids)	Longer (13–18 amino acids)
Co-receptor Interaction	α₃ domain binds CD8	β₂ domain binds CD4
Constitutive Expression	Nearly all nucleated cells	Professional APCs (DCs, Macrophages, B cells)
Inducible Expression	Upregulated by IFN-α, IFN-β, IFN-γ	Induced on non-APCs by IFN-γ
Nature of Presented Peptides	Endogenous (cytosolic, viral, self)	Exogenous (extracellular, phagocytosed)
Responding T Cell Subset	Cytotoxic T cells (T_C; CD8+)	Helper T cells (T_H; CD4+)
Classical Loci (Humans / HLA)	HLA-A, HLA-B, HLA-C	HLA-DR, HLA-DQ, HLA-DP
Classical Loci (Mice / H-2)	H-2K, H-2D, H-2L	I-A, I-E

9. Antigen Processing and Presentation Pathways

T lymphocytes cannot recognize free, native antigens. Instead, they exhibit MHC Restriction:

CD8+ T_C cells recognize peptide antigens only when presented in complex with Class I MHC molecules.
CD4+ T_H cells recognize peptide antigens only when presented in complex with Class II MHC molecules.

The pathway utilized to process and load protein antigens onto MHC molecules depends on whether the antigen originates inside (endogenous) or outside (exogenous) the host cell.

The Endogenous Pathway (MHC Class I Presentation)

This pathway processes intracellular antigens, such as viral proteins synthesised during infection, intracellular bacterial proteins, or mutated self-proteins in cancer cells.

Processing Steps

Antigen Ubiquitination: Cytosolic proteins targeted for degradation are covalently tagged with multiple molecules of ubiquitin (polyubiquitination), a small, regulatory protein.
Proteasomal Degradation: The ubiquitinated proteins are threaded into the proteasome — a large, multi-subunit cylindrical protease complex. The proteasome cleaves the proteins into short peptide fragments.
Active Transport via TAP: The generated peptide fragments in the cytosol are actively transported across the rough endoplasmic reticulum (RER) membrane into the ER lumen by TAP (Transporter associated with antigen processing) — a membrane-bound heterodimer of TAP1 and TAP2 belonging to the ATP-Binding Cassette (ABC) transporter superfamily. TAP preferentially transports peptides of 5 to 10 amino acids with hydrophobic or basic carboxyl-terminal residues.
Chaperone-Assisted Assembly in the ER: A newly synthesised Class I α chain initially binds the membrane-bound chaperone calnexin, which keeps the chain partially folded. Upon binding of β₂-microglobulin (β₂m) to the α chain, calnexin dissociates, and the heterodimer associates with a multi-protein Peptide-Loading Complex (PLC) — calreticulin, ERp57, and tapasin — which bridges the Class I molecule to the TAP transporter, holding the empty cleft close to incoming peptides.
Peptide Loading and Egress: Once a high-affinity peptide entering via TAP binds the cleft, a conformational change releases the fully assembled MHC Class I–peptide complex from the PLC. The complex exits the ER via transport vesicles, undergoes carbohydrate modification in the Golgi, and is transported to the plasma membrane for presentation to CD8+ T_C cells.

The Exogenous Pathway (MHC Class II Presentation)

This pathway processes extracellular antigens (bacteria, parasites, extracellular toxins, or foreign proteins) that enter the cell via endocytosis, phagocytosis, or pinocytosis.

Processing Steps

Antigen Internalisation: Extracellular antigens are internalised by professional APCs into membrane-bound endocytic vesicles.
Endosomal Acidification and Proteolysis: These endosomes fuse with lysosomes, forming increasingly acidic compartments. The drop in pH activates hydrolytic, acid-dependent proteases, primarily cathepsins, which degrade the foreign proteins into peptide fragments of 13 to 18 amino acids.
Biosynthesis and Invariant Chain Shielding: Concurrently, Class II α and β chains are synthesised in the RER. To prevent empty Class II molecules from prematurely binding endogenous peptides in the ER lumen, the newly formed heterodimer binds a specialised, trimeric protein called the Invariant Chain (Ii, or CD74), which physically shields the peptide-binding cleft and carries sorting signals that route the complex through the Golgi to the endosomal pathway.
CLIP Formation: As the Class II–Ii complex moves into the acidic endosomal pathway (late endosomes/MIIC compartment), proteolytic enzymes cleave the invariant chain in a stepwise fashion, leaving only a small fragment — CLIP (class II-associated invariant chain peptide) — lodged within the peptide-binding cleft.
Peptide Exchange Catalysed by HLA-DM: Because CLIP has low affinity for the cleft, it must be removed to allow antigenic peptides to bind. HLA-DM — a highly conserved, non-classical, non-polymorphic Class II molecule — binds the Class II MHC–CLIP complex, inducing a conformational change that releases CLIP, after which an antigenic peptide with appropriate anchor residues binds the stabilized, empty cleft.
Regulation by HLA-DO: A second non-classical Class II molecule, HLA-DO, acts as a negative regulator. It binds HLA-DM and inhibits its catalytic activity, modulating the rate of antigen presentation under non-inflammatory conditions.
Surface Expression: Once the antigenic peptide is securely loaded, the MHC Class II–peptide complex is transported in vesicles to the plasma membrane, where it is presented to CD4+ T_H cells.

10. Presentation of Non-Peptide Antigens: The CD1 Family

While classical MHC molecules present strictly protein-derived peptide antigens, T cells can also recognize non-protein antigens (such as lipids, glycolipids, and phospholipids):

Core Definition

The CD1 Family is a group of non-classical, MHC-like transmembrane glycoproteins.

Structural Characteristics: Structurally, CD1 molecules resemble MHC Class I molecules, consisting of a heavy chain that associates non-covalently with β₂-microglobulin.
Functional Characteristics: Functionally, CD1 molecules behave more like MHC Class II. They localize to endocytic compartments, where they capture and load exogenous lipid antigens (such as mycolic acid from Mycobacterium tuberculosis) that have been internalized by the cell.
Target Lymphocytes: CD1-lipid complexes are presented primarily to Natural Killer T (NKT) cells and specific γδ T-cell subsets, facilitating rapid responses to lipid-rich cell walls of pathogens.

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