Adaptive Immunity and Antigen Presentation: Key Concepts in Immunology

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity is a specific defense mechanism that develops after exposure to pathogens. It is characterized by its ability to recognize specific antigens and provide long-lasting protection through the formation of memory cells.

• Specificity: Targets particular pathogens or foreign molecules.

• Memory: Generates memory cells for faster response upon re-exposure.

• Long-lasting protection: Immunological memory ensures sustained defense.

• Example: Vaccination induces adaptive immunity against diseases like measles.

Antigen Presentation and Antigen Presenting Cells (APCs)

Antigen Presenting Cells (APCs)

APCs are specialized cells that process and present antigens to T cells, initiating adaptive immune responses.

• Dendritic cells: Efficiently capture and present antigens to T cells.

• Macrophages: Engulf pathogens and display antigens to T cells.

• B lymphocytes: Produce antibodies and present antigens to helper T cells.

• Example: Dendritic cells activate naïve T cells in lymph nodes.

Antigen Presentation

Antigen presentation is the process by which protein antigens are displayed on cell surfaces bound to MHC molecules for recognition by T cells.

• Key step: Enables T cell activation and immune response.

• Example: Viral peptides presented by infected cells to cytotoxic T cells.

Major Histocompatibility Complex (MHC)

MHC Overview

The Major Histocompatibility Complex (MHC) is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules.

• MHC Class I: Found on all nucleated cells; presents endogenous antigens to CD8+ T cells.

• MHC Class II: Found on APCs; presents exogenous antigens to CD4+ T cells.

• Example: MHC molecules are critical for organ transplant compatibility.

MHC Class I vs. MHC Class II

Feature	MHC Class I	MHC Class II
Expression	All nucleated cells	APCs (dendritic cells, macrophages, B cells)
Antigen Source	Endogenous (intracellular)	Exogenous (extracellular)
Recognized by	CD8+ T cells	CD4+ T cells

Co-receptors and T Cell Recognition

CD4 and CD8

CD4 and CD8 are co-receptors on T cells that determine their function and the type of MHC molecule they recognize.

• CD4: Found on helper T cells; recognizes antigens presented by MHC class II.

• CD8: Found on cytotoxic T cells; recognizes antigens presented by MHC class I.

• Example: CD8+ T cells kill virus-infected cells.

T Cell Receptors (TCRs)

TCRs are molecules on T cells that specifically recognize antigens presented by MHC molecules.

• Specificity: Each T cell expresses a unique TCR.

• Activation: TCR binding to MHC-antigen complex triggers T cell response.

Peptide Binding and Anchor Residues

Peptide Binding

Peptide binding refers to the noncovalent interaction between antigenic peptides and MHC molecules, allowing antigen presentation.

• Stability: Peptide-MHC complex stability is crucial for T cell recognition.

Anchor Residues

Anchor residues are specific amino acids in a peptide that bind to pockets in the MHC molecule to stabilize the complex.

• Importance: Determines which peptides can be presented by a given MHC allele.

Antigen Processing Pathways

Proteasome and TAP

The proteasome and TAP are involved in processing and transporting peptides for MHC class I presentation.

• Proteasome: Degrades intracellular proteins into peptides.

• TAP: Transports peptides into the endoplasmic reticulum for loading onto MHC class I.

• Equation:

$\text{Protein} \xrightarrow{\text{Proteasome}} \text{Peptide} \xrightarrow{\text{TAP}} \text{ER} \rightarrow \text{MHC I-Peptide Complex}$

Invariant Chain, CLIP, and HLA-DM

These molecules regulate MHC class II antigen presentation.

• Invariant chain: Blocks peptide-binding groove of MHC II until it reaches the endosome.

• CLIP: Fragment of invariant chain occupying MHC II groove until replaced.

• HLA-DM: Facilitates exchange of CLIP for antigenic peptides in MHC II.

• Equation:

$\text{MHC II-Invariant Chain} \rightarrow \text{MHC II-CLIP} \xrightarrow{\text{HLA-DM}} \text{MHC II-Peptide Complex}$

Cross-Presentation

Definition and Importance

Cross-presentation is a process where dendritic cells present extracellular antigens on MHC class I molecules to CD8+ T cells.

• Significance: Allows immune response against viruses and tumors that do not infect APCs directly.

• Example: Dendritic cells cross-present viral antigens to activate cytotoxic T cells.

Summary Table: Key Terms and Functions

Term	Function
Adaptive immunity	Specific, long-lasting immune response with memory
APCs	Present antigens to T cells
MHC	Displays antigens for T cell recognition
CD4	Helper T cell co-receptor for MHC II
CD8	Cytotoxic T cell co-receptor for MHC I
Proteasome	Degrades proteins for MHC I presentation
TAP	Transports peptides into ER for MHC I
Invariant chain	Blocks MHC II groove until endosome
CLIP	Occupies MHC II groove until replaced
HLA-DM	Replaces CLIP with antigenic peptide
Cross-presentation	Extracellular antigens presented on MHC I