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Affinity and Avidity

Affinity and Avidity

Antigens Involved in Adaptive Immunity

Antigens Involved in Adaptive Immunity

An antigen is any substance the immune system identifies as foreign and potentially harmful to the body, prompting an immune response. Antigens have two functional properties: immunogenicity and reactivity. Immunogenicity is the ability of an antigen to stimulate a specific immune response. At the same time, reactivity describes the antigen's ability to react with the cells and antibodies produced in response to it.
Complete Antigens
Complete antigens possess both immunogenicity and reactivity.

Antigen Processing Pathways

Antigen Processing Pathways

MHC molecules are key players in the immune response, enabling T cells to recognize and respond to specific antigens. They are present on the surface of all nucleated cells in the body and are instrumental in presenting antigens to T cells and activating them. T cells recognize the MHC-antigen complex and initiate an immune response. MHC class I and MHC class II are two main types of MHC molecules, each associated with a distinct antigen processing pathway.
MHC Class I: Presenting Endogenous...

T Cell Activation and Clonal Selection

T Cell Activation and Clonal Selection

T cells are integral to our adaptive immune system, recognizing and effectively responding to foreign antigens. T cell activation and clonal selection are pivotal in orchestrating this immune response. This article elucidates these mechanisms, detailing the roles of cluster of differentiation (CD) markers, major histocompatibility complex (MHC) molecules, costimulatory signals, and the process of clonal selection.
Naive T cells that have not yet encountered an antigen express two primary CD...

T Cell Types and Functions

T Cell Types and Functions

When T cells with CD4 markers are activated, they give rise to two types of effector cells: helper T cells and regulatory T cells. Meanwhile, T cells with CD8 markers differentiate into effector cytotoxic T cells. The differentiation of CD4 T cells into helper T cell subsets, such as Th1, Th2, and Th17 cells, is dependent on the antigen type, antigen-presenting cell, and regulatory cytokines.
Th1 cells stimulate dendritic cells to express necessary co-stimulatory molecules on their surfaces for...

B Cell Activation and Differentiation

B Cell Activation and Differentiation

The adaptive immune response, a sophisticated defense mechanism, relies on the activation and differentiation of B lymphocytes, or B cells. These processes enable our bodies to mount a tailored response against specific pathogens such as bacteria, free virus particles, toxins, and parasites.
When naive B cells encounter a specific antigen that can bind to the B cell receptor (BCR) on their surface, they undergo sensitization to respond to the antigen's presence. Sensitization begins with...

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Related Experiment Video

Updated: Jun 28, 2026

Visualizing Antigen Specific CD4+ T Cells using MHC Class II Tetramers

Visualizing Antigen Specific CD4+ T Cells using MHC Class II Tetramers

Published on: March 6, 2009

Epitope affinity for MHC class I determines helper requirement for CTL priming.

A Franco¹, D A Tilly, I Gramaglia

¹Division of Immunochemistry, La Jolla Institute for Allergy and Immunology, San Diego, CA 92121, USA. alessandra_franco@liai.org

Nature Immunology

|March 15, 2001

Summary

Antigen-specific CD8+ cytotoxic T lymphocytes (CTL) priming requires T helper cell help for low-affinity immunogens, but not for high-affinity ones. The efficiency of help varied depending on the specific epitope studied.

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Use of Single Chain MHC Technology to Investigate Co-agonism in Human CD8+ T Cell Activation

Use of Single Chain MHC Technology to Investigate Co-agonism in Human CD8+ T Cell Activation

Published on: February 28, 2019

Area of Science:

Immunology
T cell biology
Vaccine development

Background:

CD8+ cytotoxic T lymphocytes (CTLs) are crucial for adaptive immunity against viral infections and cancer.
T cell help, typically provided by CD4+ T helper cells, is often considered essential for robust CTL responses.
The role of T cell help in CTL priming can be influenced by the affinity of the immunogen for MHC class I molecules.

Purpose of the Study:

To investigate the requirement of T cell help for the priming and memory generation of antigen-specific CD8+ CTLs.
To determine how immunogen affinity for MHC class I molecules impacts the need for T cell help.
To compare the efficacy of different sources of T cell help when required.

Main Methods:

Studied three optimal-length CTL epitopes with high affinity for the Kb restriction element.
Investigated two subdominant CTL epitopes with intermediate MHC binding affinity.
Assessed CTL priming with and without T helper cell intervention (via MHC class II-restricted epitope or anti-CD40 antibody).

Main Results:

High-affinity immunogens induced significant CTL priming and memory generation without T cell help.
Subdominant epitopes with intermediate affinity required either MHC class II-restricted T helper cell epitope or CD40 blockade for effective CTL priming.
The efficiency of T cell help varied between the two tested sources, depending on the specific epitope.

Conclusions:

High-affinity binding to MHC class I molecules can render CTL priming independent of T cell help.
T cell help becomes essential for CTL responses when immunogens have intermediate affinity for MHC class I.
The choice of T cell help strategy can significantly impact the magnitude of the CTL response.