Functions of T Lymphocytes: Videos & Practice Problems

Cytotoxic T cells, also known as TC cells, play a crucial role in the immune response by targeting and eliminating infected host cells. These cells are particularly effective against cells presenting intracellular pathogens, such as viruses, on their MHC class I molecules. When a cytotoxic T cell encounters an infected cell displaying immunogenic microbial antigens, it initiates a process that leads to apoptosis, or programmed cell death, of the infected cell.

Healthy, uninfected cells present non-immunogenic self-made peptides on their MHC class I molecules, which do not trigger an immune response. This distinction is vital, as cytotoxic T cells become anergic, or unresponsive, when they interact with these healthy cells. In contrast, infected cells present antigens that elicit an immune response, allowing cytotoxic T cells to recognize and respond to the threat.

Upon binding to an infected cell, cytotoxic T cells release various proteins, including proteases and perforin. Proteases are enzymes that degrade cellular proteins, while perforin forms pores in the infected cell's membrane. These pores facilitate the entry of proteases, leading to the degradation of essential proteins within the infected cell, ultimately inducing apoptosis. This mechanism allows for the controlled elimination of infected cells without exposing pathogens to surrounding healthy cells.

Additionally, cytotoxic T cells can secrete cytokines, which are signaling molecules that enhance the immune response by activating macrophages and increasing antigen presentation on dendritic cells. This communication amplifies the overall immune response, ensuring a robust defense against pathogens.

In summary, cytotoxic T cells are essential for identifying and eliminating infected cells through apoptosis, thereby controlling the spread of intracellular pathogens while maintaining the integrity of healthy cells. Understanding the functions of these cells is fundamental to grasping the broader mechanisms of the immune system.

Helper T cells, or TH cells, play a crucial role in the immune response by differentiating into various subtypes based on signals from dendritic cells. Each subtype produces specific cytokines that help regulate immune responses in distinct ways. Among the notable subtypes are TH1, TH2, and TH17 cells, each tailored to combat different types of pathogens.

TH1 cells are primarily involved in responding to intracellular pathogens, such as viruses. They activate macrophages and cytotoxic T cells (TC cells), which are essential for targeting and eliminating these intracellular threats. This activation is vital for a robust immune response against infections that occur within host cells.

In contrast, TH2 cells respond to multicellular pathogens, particularly helminths (parasitic worms). They facilitate the recruitment and activation of eosinophils and basophils, which are specialized white blood cells that target and eliminate these larger, multicellular invaders.

TH17 cells are adept at responding to extracellular pathogens, such as bacteria that reside outside of cells. They activate neutrophils, which are key players in the immune system's defense against these external threats, helping to clear infections effectively.

While this overview highlights the distinct functions of TH1, TH2, and TH17 cells, it is important to recognize that all helper T cell subtypes work collectively to orchestrate a comprehensive immune response. As we progress in our studies, we will explore the functions of these cells in greater detail, emphasizing their collaborative roles in maintaining immune health.

Helper T cells, also known as T_H cells, play a crucial role in the immune system by activating other immune cells, particularly macrophages. Initially, naive or inactive helper T cells become activated when they encounter dendritic cells that present antigens on MHC class II molecules. Unlike cytotoxic T cells, which induce apoptosis in infected cells, helper T cells primarily function by producing cytokines—signaling molecules that enhance the immune response.

Macrophages are essential immune cells that continuously engulf, degrade, and process pathogens. They can also present these pathogens as antigens on their own MHC class II molecules. When effector helper T cells bind to these antigens, they become activated and release cytokines that further stimulate macrophages. This interaction significantly boosts the macrophages' immune capabilities, enabling them to produce more lysosomes—organelles that aid in the destruction of pathogens—and increase the production of antimicrobial substances that effectively eliminate invaders.

The activation process can be visualized as follows: an unactivated macrophage engulfs a pathogen, processes it, and presents the resulting antigens on its surface via MHC class II molecules. When a helper T cell recognizes these antigens, it releases cytokines that activate the macrophage. This activation enhances the macrophage's ability to destroy pathogens, resulting in a more robust immune response.

In summary, helper T cells are vital for enhancing the immune response by activating macrophages and other immune cells through cytokine signaling. This collaboration improves the overall effectiveness of the immune system in combating infections.