Short Answer
Understanding why the immune system can recognise infections inside a cell as well as microbes that remain outside the cell hinges on two families of major histocompatibility complex (MHC) molecules: MHC class I and MHC class II. Both families bind short peptide fragments and display them on the cell surface, but they differ in where the peptides come from, how they are processed, which cells express them, and which T‑cell subset they engage. This article compares the two pathways, explains the underlying mechanisms, and provides concrete examples to cement the concepts.
Quick Overview
- MHC class I: presents endogenous (intracellular) peptides to CD8⁺ cytotoxic T lymphocytes (CTLs).
- MHC class II: presents exogenous (extracellular) peptides to CD4⁺ helper T lymphocytes (Th cells).
Cellular Distribution
All nucleated cells in the body express MHC class I molecules on their plasma membrane. In contrast, MHC class II molecules are restricted to professional antigen‑presenting cells (APCs) – dendritic cells, macrophages, B cells – and can be induced on some other cell types during inflammation.
Source: British Society for Immunology describes the expression patterns of both classes of MHC molecules [1].
Source of Antigenic Peptides
Endogenous pathway (MHC I) – Peptides originate from proteins synthesized inside the cell, such as viral proteins, tumor antigens, or normal self‑proteins that are turned over by the proteasome.
Exogenous pathway (MHC II) – Peptides are derived from proteins that the cell has taken up from the extracellular environment by endocytosis or phagocytosis, such as bacterial toxins, extracellular parasites, or soluble antigens.
These sources are the fundamental distinction highlighted in the immunology literature [1][2].
Processing Compartments and Enzymes
Both pathways involve proteolysis, peptide loading, and transport to the cell surface, but they occur in different intracellular organelles.
1. MHC Class I Processing
- Proteasomal degradation: Cytosolic proteins are chopped into 8–10‑amino‑acid fragments by the 26S proteasome.
- Transport into the endoplasmic reticulum (ER): Peptides are shuttled by the transporter associated with antigen processing (TAP) into the ER lumen.
- Peptide trimming and loading: ER aminopeptidases (ERAP1/2) trim peptides to the optimal length, and the peptide‑loading complex (PLC) assists binding to nascent MHC I heavy chains that have already associated with β₂‑microglobulin.
- Transport to the plasma membrane: The stable peptide‑MHC I complex exits the ER via the Golgi and is displayed on the cell surface.
These steps are summarized in the British Society for Immunology overview of the MHC I pathway [1].
2. MHC Class II Processing
- Endocytosis or phagocytosis: Extracellular proteins are internalised into early endosomes.
- Acidification and proteolysis: The endosomal compartment matures into a late endosome/lysosome where acidic proteases (cathepsins) cleave proteins into 12–25‑amino‑acid peptides.
- Invariant chain removal: Newly synthesized MHC II α‑ and β‑chains first bind an invariant chain (Ii) in the ER, which blocks the peptide‑binding groove. The MHC‑Ii complex traffics to the endosome, where Ii is degraded, leaving the class II‑associated invariant chain peptide (CLIP) in the groove.
- Peptide exchange: The chaperone HLA‑DM (or H2‑M in mice) catalyses removal of CLIP and facilitates loading of high‑affinity exogenous peptides.
- Surface expression: The peptide‑MHC II complex travels to the plasma membrane for presentation to CD4⁺ T cells.
OpenStax outlines these steps for the MHC II pathway and notes that the process occurs primarily in professional APCs [2].
Outcome: Which T‑Cell Subset Is Engaged?
When a T‑cell receptor (TCR) on a CD8⁺ T cell recognises a peptide‑MHC I complex, the CD8 co‑receptor stabilises the interaction, leading to activation of the cytotoxic program – release of perforin, granzymes, and cytokines that kill the presenting cell.
When a TCR on a CD4⁺ T cell recognises a peptide‑MHC II complex, the CD4 co‑receptor binds the MHC II molecule, triggering helper functions: cytokine secretion that activates B cells, macrophages, and other immune cells.
Both pathways are essential for a coordinated adaptive response, as described in the NCBI Immunobiology chapter on antigen presentation [3].
Special Cases: Cross‑Presentation and Autophagy
Although the textbook distinction is clear, biology provides exceptions:
- Cross‑presentation: Certain dendritic cells can load exogenous antigens onto MHC I molecules, allowing CD8⁺ T cells to respond to extracellular pathogens or tumor antigens. This bridges the two pathways.
- Autophagy‑mediated MHC II presentation: Endogenous proteins can be delivered to the MHC II loading compartment via autophagosomes, enabling presentation of intracellular antigens on MHC II.
Both phenomena are mentioned in the British Society for Immunology article as extensions of the core pathways [1].
Comparison Table
| Feature | MHC Class I | MHC Class II |
|---|---|---|
| Expressed on | All nucleated cells | Professional APCs (dendritic cells, macrophages, B cells) and induced cells |
| Source of peptide | Endogenous (cytosolic) proteins | Exogenous (extracellular) proteins |
| Key processing organelle | Proteasome → TAP → ER | Endosome/lysosome |
| Typical peptide length | 8–10 aa | 12–25 aa |
| Co‑receptor on T cell | CD8 | CD4 |
| Primary T‑cell response | CD8⁺ cytotoxic T cells | CD4⁺ helper T cells |
| Typical pathogens detected | Viruses, intracellular bacteria, tumour antigens | Extracellular bacteria, parasites, toxins |
Concrete Example: Viral Infection vs. Bacterial Extracellular Infection
Scenario 1 – Influenza virus infection: Infected epithelial cells synthesize viral proteins in the cytoplasm. These proteins are degraded by the proteasome, peptides are transported into the ER by TAP, loaded onto MHC I, and displayed. CD8⁺ T cells that recognise a viral peptide‑MHC I complex proliferate and kill the infected cell, limiting viral spread.
Scenario 2 – Streptococcus pneumoniae extracellular infection: Macrophages ingest the bacteria by phagocytosis. Bacterial proteins are broken down in phagolysosomes, loaded onto MHC II, and presented on the macrophage surface. CD4⁺ Th cells recognizing the bacterial peptide‑MHC II complex become activated, secrete cytokines (e.g., IFN‑γ), and help B cells produce specific antibodies that opsonise the bacteria for clearance.
These contrasting scenarios illustrate why the immune system needs both pathways.
Why the Distinction Matters
Clinical and vaccine design rely on manipulating these pathways. For example, live‑attenuated viral vaccines exploit the MHC I route to generate strong CD8⁺ T‑cell immunity, while protein subunit vaccines aim to stimulate CD4⁺ helper responses through MHC II presentation. Understanding the cell biology behind each pathway enables rational design of immunotherapies, such as checkpoint inhibitors that boost CD8⁺ T‑cell activity against tumours.
Check Your Understanding
Which of the following statements is true?
- All cells can present exogenous antigens on MHC I.
- MHC II peptides are typically 8–10 amino acids long.
- Cross‑presentation allows dendritic cells to activate CD8⁺ T cells with extracellular antigens.
Answer: 3. Cross‑presentation is the exception that enables CD8⁺ T‑cell activation by extracellular antigens, as noted in the cross‑presentation description [1].
Next Steps in Learning
Having grasped the core differences between MHC class I and II pathways, the logical next topic is how T‑cell activation is translated into effector functions, including the roles of co‑stimulatory molecules and cytokine signalling.
FAQ
What types of cells express MHC class I molecules?
All nucleated cells in the body constitutively express MHC class I molecules on their plasma membrane, allowing them to present endogenous peptide fragments to CD8⁺ cytotoxic T cells.
Which cells are considered professional antigen‑presenting cells for MHC class II?
Professional APCs include dendritic cells, macrophages, and B cells. These cells express high levels of MHC class II and can present exogenous peptides to CD4⁺ helper T cells. During inflammation, other cell types can be induced to express MHC II.
How does the transporter associated with antigen processing (TAP) function in the MHC I pathway?
TAP is an ATP‑dependent transporter located in the endoplasmic reticulum membrane. It translocates peptide fragments generated by the cytosolic proteasome into the ER lumen, where they are trimmed by ERAP1/2 and loaded onto nascent MHC class I molecules.
What is the role of the invariant chain (Ii) in MHC class II antigen presentation?
The invariant chain binds to the peptide‑binding groove of newly synthesized MHC class II α and β chains in the ER, preventing premature peptide loading. The MHC‑Ii complex traffics to endosomal compartments, where proteolysis removes Ii, leaving CLIP, which is then exchanged for antigenic peptide with the help of HLA‑DM.
How does cross‑presentation enable CD8⁺ T‑cell responses to extracellular antigens?
Cross‑presentation occurs primarily in certain dendritic cells that can internalize exogenous proteins, process them via the proteasome, and load the resulting peptides onto MHC class I molecules. This pathway bridges the extracellular and intracellular antigen‑presentation systems, allowing CD8⁺ T cells to recognize pathogens that do not infect the presenting cell directly.
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