Activation of T Lymphocytes: Videos & Practice Problems

T lymphocytes are activated by antigen-presenting cells (APCs) like dendritic cells, which present harmful antigens using MHC class 1 or class 2. This triggers the production of co-stimulatory molecules, leading to T cell activation, proliferation, and differentiation into effector or memory T cells. Conversely, when harmless antigens are presented, no co-stimulatory signals are produced, resulting in T cells becoming anergic and undergoing apoptosis. This mechanism ensures the immune system targets only dangerous antigens, maintaining immune tolerance to harmless substances.

T-Cell Activation

The activation of T lymphocytes, or T cells, is a crucial process in the immune response, primarily facilitated by antigen presenting cells (APCs) such as dendritic cells. These APCs possess both classes of major histocompatibility complexes (MHCs), specifically MHC class I and MHC class II, allowing them to activate different types of T cells. MHC class I is responsible for activating naive cytotoxic T cells, while MHC class II activates naive helper T cells.

Upon activation, T cells undergo proliferation, creating numerous identical clones that can differentiate into effector T cells or memory T cells. This differentiation is essential for mounting an effective immune response against pathogens. Dendritic cells play a pivotal role in this process by presenting antigens, which can be classified into two major types: harmful (immunogenic) and harmless (non-immunogenic) antigens.

When a dendritic cell presents a harmful antigen, it also expresses co-stimulatory molecules on its surface. These molecules serve as signals that communicate the danger of the antigen to naive T cells, enhancing their activation. In contrast, if a dendritic cell presents a harmless antigen, it does not produce co-stimulatory molecules. Consequently, naive T cells exposed to these harmless antigens do not become activated; instead, they enter a state known as anergy, meaning they become unresponsive. Anergic T cells eventually undergo apoptosis, effectively eliminating the potential for an immune response against harmless antigens.

This mechanism is vital for maintaining immune tolerance, ensuring that the immune system does not react to non-threatening substances, thereby preventing unnecessary immune responses that could lead to tissue damage or autoimmune disorders. Understanding the activation of T lymphocytes and the role of dendritic cells in distinguishing between harmful and harmless antigens is fundamental to immunology and the development of therapeutic strategies for various diseases.

Antigen presenting cells, including dendritic cells in lymph nodes, are observed to bind T cells on their surfaces. If the dendritic cell is presenting harmful antigens on its surface and creating co-stimulatory molecules, what will happen to the T cells that bind to the dendritic cell?

• A. The naive T cells that bind the dendritic cell will activate and form effector T cells.
• B. The cytotoxic T cells which bind the dendritic cell will signal the dendritic cell to undergo apoptosis.
• C. The anergic T cells which bind the dendritic cell will undergo apoptosis.
• D. The helper T cells that bind the dendritic cell will deactivate and become naive T cells.

Answer: (A)

Comic Strip for T-Cell Activation

T cell activation is a crucial process in the immune response, primarily involving dendritic cells and their interaction with T lymphocytes. This interaction can be understood through a comic strip analogy that illustrates two scenarios: the presentation of harmful antigens and harmless antigens by dendritic cells.

In the first scenario, when a dendritic cell presents a harmful antigen, such as a virus, it triggers the activation of T lymphocytes. The dendritic cell, depicted as a character in the comic, produces co-stimulatory molecules in response to the dangerous antigen. This activation is essential for the helper T cell to recognize the threat and initiate an immune response. The helper T cell, upon recognizing the harmful antigen, mobilizes additional immune defenses to eliminate the threat.

Conversely, in the second scenario, when a dendritic cell presents a harmless antigen, the helper T cell does not become activated. In this case, the dendritic cell presents a self-cell, which is recognized as non-threatening. The helper T cell identifies this harmless antigen and remains unresponsive, leading to a state known as anergy. Anergic T cells are essentially inactive and may undergo apoptosis, a programmed cell death, which helps prevent the immune system from mistakenly targeting healthy cells.

This differentiation between harmful and harmless antigens is vital for maintaining immune tolerance and ensuring that the immune system effectively targets only dangerous pathogens while sparing the body's own cells. Understanding this process is fundamental for grasping how the immune system operates and the importance of T cell activation in immune responses.

The role of dendritic cells is to:

• A. Activate B cells via antigen presentation.
• B. Present antigens on MHC I only.
• C. Activate the complement system classical pathway.
• D. Perform phagocytosis.
• E. Activate T helper & T cytotoxic cells via MHC I & II.

Answer: (E)

Before T cells become activated, they require an antigen encounter. Naive T cells are shown to express L-selectin which aids in binding to high endothelial cells expressing L-selectin ligand in regions where they are most likely to encounter an antigen. In which of the following locations are these high endothelial cells most likely to be located?

• A. Bone marrow.
• B. Capillaries.
• C. Lymph nodes.
• D. Heart.

Answer: (C)

Here’s what students ask on this topic:

What are the roles of MHC class 1 and MHC class 2 in T cell activation?

MHC class 1 and MHC class 2 molecules play crucial roles in T cell activation by presenting antigens to T cells. MHC class 1 molecules present endogenous antigens (from within the cell) to cytotoxic T cells (CD8+), leading to their activation. On the other hand, MHC class 2 molecules present exogenous antigens (from outside the cell) to helper T cells (CD4+), which then activate and coordinate the immune response. The interaction between the T cell receptor (TCR) and the MHC-antigen complex, along with co-stimulatory signals, is essential for the activation, proliferation, and differentiation of T cells into effector and memory cells.

How do co-stimulatory molecules influence T cell activation?

Co-stimulatory molecules are essential for the full activation of T cells. When a dendritic cell presents a harmful antigen on its MHC molecules, it also expresses co-stimulatory molecules on its surface. These molecules provide a second signal that confirms the presence of a dangerous antigen. The interaction between co-stimulatory molecules on the dendritic cell and their receptors on the T cell ensures that the T cell becomes fully activated. Without this co-stimulatory signal, the T cell remains anergic (unresponsive) and eventually undergoes apoptosis, preventing an immune response to harmless antigens.

What happens to T cells when they encounter harmless antigens?

When T cells encounter harmless antigens presented by dendritic cells, no co-stimulatory molecules are produced. As a result, the T cells do not receive the necessary second signal for activation. Instead of becoming activated, these T cells become anergic, meaning they become unresponsive. Anergic T cells eventually undergo apoptosis, which is a programmed cell death process. This mechanism ensures that the immune system does not mount a response against harmless antigens, maintaining immune tolerance and preventing unnecessary or harmful immune reactions.

What is the significance of T cell differentiation into effector and memory cells?

Upon activation, T cells proliferate and differentiate into effector and memory cells. Effector T cells are responsible for immediate immune responses: cytotoxic T cells (CD8+) kill infected cells, while helper T cells (CD4+) coordinate the immune response by activating other immune cells. Memory T cells, on the other hand, provide long-term immunity. They persist in the body after the initial infection has been cleared and can respond more rapidly and effectively if the same antigen is encountered again. This differentiation ensures both an immediate defense and long-term protection against pathogens.

What is the role of dendritic cells in T cell activation?

Dendritic cells are crucial antigen-presenting cells (APCs) that play a key role in T cell activation. They capture and process antigens, then present them on their surface using MHC class 1 or MHC class 2 molecules. When presenting harmful antigens, dendritic cells also express co-stimulatory molecules, which are necessary for the full activation of naive T cells. This interaction leads to the proliferation and differentiation of T cells into effector and memory cells. Dendritic cells thus serve as a bridge between the innate and adaptive immune systems, initiating and regulating the adaptive immune response.