Immunology Lesson: Antigen Processing, MHC Types & T Cells

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Updated: May 20, 2025

Lesson

Lesson Overview

What Is Immunology and Why Is It Important?
How Do T Cells Recognize and Respond to Antigens?
Where Are MHC Molecules Located and What Do They Do?
How Does the Immune System Process Intracellular Antigens?
How Are Extracellular Antigens Processed for Immune Activation?
What Is the Role of the Invariant Chain in MHC Class II Processing?
How Does the TAP Transporter Facilitate Class I MHC Function?
How Do Genetic Defects in TAP Affect Immune Function?
How Do Viruses Escape Detection by the Immune System?
How Do the Cytosolic and Endocytic Pathways Compare?
Why Does Antigen Presentation Matter in Immunology?

When Samantha blanked out on how T cells identify infected cells during her biology exam, she realized memorizing terms wasn't enough. Understanding immunology means tracing how antigens are processed and matched with the right T cells. This lesson breaks it all down simply so you can learn it once and learn it well.

What Is Immunology and Why Is It Important?

This section introduces immunology as the science of immune protection, surveillance, and regulation in living organisms.

Immunology is the scientific discipline that studies how the immune system defends the body against harmful pathogens such as bacteria, viruses, fungi, and parasites. Immunology focuses on identifying how immune cells recognize foreign substances, eliminate them, and develop memory for future protection. Immunological knowledge is vital in understanding:

Disease mechanisms (e.g., infections, autoimmunity)
Vaccine development
Transplantation compatibility
Allergies and hypersensitivities
Immunodeficiencies (e.g., HIV/AIDS)

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Immunology Multiple Choice Questions

How Do T Cells Recognize and Respond to Antigens?

This section explores the role of T lymphocytes and how their receptors interact with MHC-peptide complexes.

T cells are specialized white blood cells that mediate adaptive immune responses. T cells detect antigens through T-cell receptors (TCRs), but only when antigens are processed and presented on MHC (Major Histocompatibility Complex) molecules.

CD4+ T cells recognize peptides presented by MHC Class II molecules on antigen-presenting cells.
CD8+ T cells recognize peptides on MHC Class I molecules present on all nucleated cells.

This MHC-restricted recognition ensures that:

CD4+ T cells assist extracellular immune responses.
CD8+ T cells eliminate infected or cancerous host cells.

Where Are MHC Molecules Located and What Do They Do?

This section identifies MHC distribution and correlates it with their respective immune functions.

MHC molecules are glycoproteins encoded by the HLA (human leukocyte antigen) gene complex. Their function and location include:

MHC Type	Expressed On	Presents Peptides From	Recognized By
Class I	All nucleated cells	Cytosolic/intracellular origin	CD8+ T lymphocytes
Class II	Dendritic cells, macrophages, B cells	Endocytic/extracellular origin	CD4+ T lymphocytes

How Does the Immune System Process Intracellular Antigens?

This section explains the cytosolic pathway for antigen processing and Class I MHC presentation.

Intracellular pathogens such as viruses generate proteins in the cytoplasm. These undergo:

Proteasomal degradation into peptides (8–11 amino acids)
Transport into the ER by TAP
Binding to MHC Class I molecules
Trafficking through the Golgi to the cell surface

This pathway enables cytotoxic CD8+ T cells to recognize and kill infected cells.

How Are Extracellular Antigens Processed for Immune Activation?

This section describes the endocytic pathway and MHC Class II-dependent presentation.

Antigens from bacteria and other extracellular microbes undergo the following process:

Uptake into endosomes via phagocytosis or receptor-mediated endocytosis
Proteolysis in lysosomes into antigenic peptides
Synthesis of MHC Class II and invariant chain in the ER
Transport of MHC II to the lysosome
Degradation of the invariant chain and peptide binding
Surface expression of the MHC-peptide complex for CD4+ T cell recognition

What Is the Role of the Invariant Chain in MHC Class II Processing?

This section details the protective and regulatory function of the invariant chain in antigen presentation.

The invariant chain (Ii) has key functions:

Binds MHC Class II in the ER to stabilize the molecule
Blocks premature peptide binding
Guides MHC Class II to the late endosome/lysosome
Is enzymatically degraded, leaving CLIP (Class II-associated invariant chain peptide)
Allows antigen peptides to replace CLIP with the help of HLA-DM

How Does the TAP Transporter Facilitate Class I MHC Function?

This section outlines TAP's role in delivering peptides from the cytoplasm into the ER for Class I presentation.

TAP (Transporter associated with Antigen Presentation):

Is a heterodimer of TAP1 and TAP2 proteins
Resides in the ER membrane
Uses ATP to move peptides (8–16 residues) into the ER
Provides peptides for loading onto MHC Class I
Is essential for efficient CD8+ T cell activation

How Do Genetic Defects in TAP Affect Immune Function?

This section describes the clinical consequences of TAP mutations and their immunological impact.

Defective TAP results in:

Low MHC Class I expression
Impaired peptide presentation
Weak CD8+ T cell activation
Increased susceptibility to recurrent infections (especially respiratory)

Clinically, this condition may resemble chronic respiratory infections with poor viral clearance.

How Do Viruses Escape Detection by the Immune System?

This section explores how viral proteins manipulate MHC expression and antigen processing.

Viruses use sophisticated evasion strategies:

Adenoviruses suppress MHC I transcription → fewer MHC I molecules
HSV blocks TAP transporter → reduced peptide availability

Consequences of immune evasion include:

Reduced antigen presentation
Impaired T cell surveillance
Persistent or latent infections

Virus	Immune Evasion Mechanism	Effect on Immune Response
Adenovirus	Inhibits MHC I gene expression	Decreases CD8+ T cell recognition
Herpes (HSV)	Blocks TAP transporter	Prevents peptide loading

How Do the Cytosolic and Endocytic Pathways Compare?

This section contrasts the two major antigen processing routes in immune defense.

Feature	Cytosolic Pathway (Class I)	Endocytic Pathway (Class II)
Pathogen Type	Intracellular (e.g., viruses)	Extracellular (e.g., bacteria)
MHC Class Used	Class I	Class II
T Cell Activated	CD8+ cytotoxic T cells	CD4+ helper T cells
Processing Compartment	Cytosol, ER	Endosome, lysosome
Transporter Required	Yes (TAP)	No
Invariant Chain Used	No	Yes
Peptide Length	8–11 amino acids	13–25 amino acids