U01.02.015 T- and B-cell activation

Understanding T- and B-cell activation is crucial for mastering immunology concepts. Below, the process is structured with detailed explanations, tables, and points to aid in preparation.

Overview of Antigen-Presenting Cells (APCs)

APCs play a central role in T- and B-cell activation. Key APCs include:

Dendritic cells (specialized for T-cell activation).
Macrophages.
B cells.
Langerhans cells (skin-resident dendritic cells).

T-cell Activation

Steps Involved in T-cell Activation

T-cell activation requires two signals:

Signal 1: Antigen recognition.
Signal 2: Co-stimulatory interaction for survival and proliferation.

Step	Details
Antigen Sampling	Dendritic cells process exogenous antigens and migrate to the draining lymph node.
Signal 1	– Exogenous antigens are presented on MHC II to CD4+ T cells. – Endogenous/cross-presented antigens on MHC I are recognized by CD8+ T cells.
Signal 2 (Co-stimulation)	B7 proteins (CD80/86) on the dendritic cell bind to CD28 on the naïve T cell.
Th Cell Activation	CD4+ T cells produce cytokines to modulate immune responses.
Tc Cell Activation	CD8+ T cells proliferate and gain the ability to kill virus-infected or abnormal cells.

Diagram of T-cell Activation

Component	Function
Dendritic Cell	Presents antigen via MHC I/II to T-cell receptor (TCR).
CD80/86 (B7)	Co-stimulatory molecules on APC that bind CD28 on T cells.
Naïve T Cell	Recognizes antigens through TCR, requires co-stimulation to activate.
Activated T Cell Actions	CD4+ T cells secrete cytokines; CD8+ T cells kill infected cells.

B-cell Activation and Class Switching

Steps in B-cell Activation

B-cell activation also requires two signals:

Signal 1: Antigen recognition and processing.
Signal 2: Interaction with activated Th cells.

Step	Details
Th Cell Activation	Occurs as described in T-cell activation (Signal 1 and Signal 2).
Signal 1	B cells bind antigen via B-cell receptor (BCR) and present it on MHC II.
Signal 2	CD40 on the B cell binds CD40L on activated Th cells.
Cytokine Release	Th cells release cytokines that guide Ig class switching.
Outcomes	B cells undergo: – Class switching. – Affinity maturation. – Antibody production.

Diagram of B-cell Activation

Component	Function
B-cell Receptor (BCR)	Mediates antigen endocytosis for MHC II presentation.
CD40-CD40L Interaction	Provides co-stimulatory signal for activation.
Activated B Cell Actions	Produces antibodies, undergoes class switching, and affinity maturation.

Key Points to Remember

APCs such as dendritic cells and macrophages are essential for T-cell activation.
Two signals are required for both T- and B-cell activation.
T-cell activation enables cytokine secretion (CD4+ cells) and cytotoxic activity (CD8+ cells).
B-cell activation leads to class switching, affinity maturation, and antibody production.
The interaction between CD40 and CD40L is pivotal for B-cell activation.

Learning Tips

Focus on Signal 1 and Signal 2 mechanisms for both T- and B-cell activation.
Use diagrams to visualize interactions like MHC-TCR binding and co-stimulatory signals.
Memorize cytokines secreted by Th cells and their effects on B cells.

References

Abbas, A. K., et al. Basic Immunology: Functions and Disorders of the Immune System.
Janeway, C. A., et al. Immunobiology: The Immune System in Health and Disease.

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