Activation of T Lymphocytes: Videos & Practice Problems

The activation of T lymphocytes involves antigen-presenting cells (APCs) like dendritic cells, which express both MHC class 1 and class 2. When presenting harmful antigens, these cells produce co-stimulatory molecules, activating naive T cells to proliferate and differentiate into effector or memory T cells. Conversely, presenting harmless antigens leads to T cell anergy, resulting in unresponsive cells that undergo apoptosis. This mechanism ensures the immune system targets only dangerous antigens, maintaining tolerance to harmless substances.

Activation of T Lymphocytes

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. This dual capability allows dendritic cells to activate either naive cytotoxic T cells through MHC class I or naive helper T cells via MHC class II.

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 a robust immune response. Dendritic cells play a pivotal role in determining the nature of the T cell response based on the type of antigen presented. There are two primary categories of antigens: harmful (immunogenic) and harmless.

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 associated with 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, which refers to a lack of response. Anergic T cells eventually undergo apoptosis, effectively removing them from the immune repertoire.

This mechanism is vital for maintaining immune tolerance, ensuring that the body does not mount an immune response against non-threatening antigens, such as normal cellular proteins. The ability of the immune system to distinguish between harmful and harmless antigens is crucial for preventing unnecessary immune reactions that could lead to tissue damage or autoimmune diseases.

In summary, the activation of T lymphocytes is a finely tuned process that relies on the interaction between dendritic cells and T cells, influenced by the nature of the antigens presented. Understanding this process is fundamental for grasping how the immune system functions and how it can be manipulated for therapeutic purposes.

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 a sweet shop, encounters a dangerous antigen, represented by "Virus VIC," which demands resources aggressively. In response to this threat, the dendritic cell produces co-stimulatory molecules that activate the helper T cell. This activation is essential for mounting an immune response to eliminate the harmful antigen. The key takeaway here is that the presence of harmful antigens leads to T cell activation, enabling the immune system to respond effectively to threats.

Conversely, in the second scenario, when a dendritic cell presents a harmless antigen, the outcome is different. The dendritic cell introduces a self-cell, referred to as "Stu," which is recognized as harmless by the helper T cell. In this case, the helper T cell does not activate because it identifies the antigen as non-threatening. Instead, the T cell becomes anergic, meaning it is unresponsive to further stimulation. This state of anergy is crucial for preventing unnecessary immune responses against harmless antigens, ultimately leading to apoptosis of these T cells. This mechanism ensures that the immune system remains focused on targeting only harmful antigens, thereby maintaining self-tolerance and preventing autoimmune reactions.

Understanding these two pathways of T cell activation is vital for grasping how the immune system differentiates between harmful and harmless entities, ensuring an appropriate response to threats while avoiding damage to the body's own tissues.

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. In contrast, MHC class 2 molecules present exogenous antigens (from outside the cell) to helper T cells (CD4+), which then activate and proliferate. This differentiation ensures that the immune response is appropriately directed, with cytotoxic T cells targeting infected or malignant cells and helper T cells coordinating the broader immune response.

How do co-stimulatory molecules influence T cell activation?

Co-stimulatory molecules are essential for the full activation of naive 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, ensuring that the T cell becomes fully activated. Without this co-stimulatory signal, the T cell remains anergic (unresponsive) and eventually undergoes apoptosis. This mechanism prevents the immune system from reacting to harmless antigens, maintaining tolerance and preventing autoimmunity.

What happens to T cells when they encounter harmless antigens?

When T cells encounter harmless antigens presented by dendritic cells, they do not receive the necessary co-stimulatory signals. As a result, these T cells become anergic, meaning they are unresponsive. Anergic T cells eventually undergo apoptosis, a process of programmed cell death. This mechanism ensures that the immune system does not mount a response against harmless antigens, such as self-proteins or benign environmental substances, thereby preventing unnecessary inflammation and autoimmunity.

What is the significance of T cell anergy in immune tolerance?

T cell anergy is crucial for maintaining immune tolerance, which is the immune system's ability to avoid attacking the body's own tissues and harmless substances. When T cells encounter harmless antigens without co-stimulatory signals, they become anergic and eventually undergo apoptosis. This process eliminates T cells that could potentially target self-antigens or benign environmental antigens, thereby preventing autoimmune diseases and ensuring that the immune response is directed only against harmful pathogens.

How do dendritic cells contribute to the activation of T lymphocytes?

Dendritic cells are key antigen-presenting cells that play a pivotal role in the activation of T lymphocytes. They capture and process antigens, presenting them on 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 dual signaling ensures that T cells are activated only in the presence of a genuine threat, leading to their proliferation and differentiation into effector and memory T cells, which are essential for an effective immune response.