T Cells: Videos & Practice Problems

T cells are crucial components of the adaptive immune system, activated by antigen presenting cells (APCs) like dendritic cells. Activation requires interaction with antigens, co-stimulatory molecules, and adhesion molecules. There are three main types of T cells: cytotoxic T cells (kill infected cells), helper T cells (activate other immune cells), and regulatory T cells (suppress immune responses). Accessory molecules CD8 and CD4 are essential for T cell function, binding to MHC class I and II molecules, respectively, facilitating effective immune responses against pathogens.

T Cells

T cells are crucial components of the adaptive immune system, primarily responsible for targeting and eliminating pathogens. Their activation is initiated by antigen presenting cells (APCs), which process pathogens and present their antigens on their surface. A key player in this process is the dendritic cell, a type of APC that interacts with T cells through specific receptors known as T cell receptors (TCRs). These receptors must recognize the presented antigens, which requires a diverse range of TCRs generated through a process called V(D)J recombination, similar to that of antibodies.

The activation of T cells occurs in the thymus, where naive T cells encounter APCs displaying antigens. For a T cell to become fully activated, it must engage with three essential components: the antigen presented by the APC, specific costimulatory molecules, and cell-to-cell adhesion molecules. If any of these interactions fail, the T cell may undergo inactivation or apoptosis, ensuring that T cells are activated only under appropriate conditions.

Once activated, T cells differentiate into various subtypes, each with distinct functions. Cytotoxic T cells (Tc) directly kill infected cells by inducing apoptosis through mechanisms such as perforin, which forms pores in target cells, or through ligands like Fas that trigger cell death. Helper T cells (Th), on the other hand, play a supportive role by activating other immune cells, including B cells and macrophages, thereby amplifying the immune response. Lastly, regulatory T cells (Treg) are essential for maintaining immune homeostasis by suppressing excessive T cell activity and preventing autoimmunity.

In addition to TCRs, T cells express accessory molecules, CD8 and CD4, which are critical for their function. CD8 is found on cytotoxic T cells and interacts with MHC class I molecules, while CD4 is present on helper and regulatory T cells, binding to MHC class II molecules. These accessory molecules stabilize the interaction between T cells and APCs, facilitating effective activation.

In summary, T cells are integral to the adaptive immune response, with their activation and differentiation being tightly regulated to ensure a precise and effective immune reaction against pathogens while preventing autoimmunity.

Cytotoxic T cells are responsible for which of the following responses?

• A. Suppressing other T cells
• B. Activating B cells
• C. Killing other infected cells
• D. Activating antigen presenting cells

Answer: (C)

Which of the following cell types is responsible for suppressing the activity of T cells?

• A. Antigen presenting cells
• B. Cytotoxic T cells
• C. Helper T cells
• D. Regulatory T cells

Answer: (D)

Here’s what students ask on this topic:

What are the main types of T cells and their functions?

There are three main types of T cells: cytotoxic T cells (Tc), helper T cells (Th), and regulatory T cells (Treg). Cytotoxic T cells directly kill infected cells by recognizing antigens presented on MHC class I molecules. They release perforin and granzymes to induce apoptosis in the target cell. Helper T cells, which express CD4, activate other immune cells like B cells, macrophages, and dendritic cells by releasing cytokines and interacting with MHC class II molecules. Regulatory T cells suppress the immune response to prevent overactivation and autoimmunity, ensuring that the immune system does not attack the body's own tissues.

How are T cells activated?

T cells are activated by antigen presenting cells (APCs) such as dendritic cells. The activation process requires three key interactions: the T cell receptor (TCR) must recognize and bind to the antigen-MHC complex on the APC, co-stimulatory molecules on the APC must engage with receptors on the T cell, and cell adhesion molecules must stabilize the interaction. If all three criteria are met, the T cell becomes activated. This process ensures that T cells are only activated in the presence of a genuine threat, preventing unnecessary immune responses.

What role do CD8 and CD4 molecules play in T cell function?

CD8 and CD4 are accessory molecules found on T cells that are crucial for their function. CD8 is found on cytotoxic T cells and binds to MHC class I molecules, helping these T cells recognize and kill infected cells. CD4 is found on helper and regulatory T cells and binds to MHC class II molecules, facilitating the activation of these T cells. These interactions are essential for the stabilization and activation of T cells, ensuring a precise and effective immune response.

What is the role of the thymus in T cell development?

The thymus is a specialized organ located in the neck and throat region, crucial for T cell development. In the thymus, immature T cells, known as thymocytes, undergo a selection process to become mature, naive T cells. This process involves positive selection, where T cells that can recognize self-MHC molecules are selected, and negative selection, where T cells that strongly react to self-antigens are eliminated. This ensures that the mature T cells are capable of recognizing foreign antigens while being tolerant to the body's own tissues.

How do antigen presenting cells (APCs) activate T cells?

Antigen presenting cells (APCs) activate T cells by processing and presenting antigens on their surface using MHC molecules. APCs, such as dendritic cells, ingest pathogens, degrade them, and present the resulting antigen fragments on MHC class I or II molecules. T cells recognize these antigen-MHC complexes through their T cell receptors (TCRs). Additionally, co-stimulatory signals and cell adhesion molecules are required to fully activate the T cells. This multi-step process ensures that T cells are only activated in the presence of a genuine threat, preventing unnecessary immune responses.