Introduction to T Lymphocytes: Videos & Practice Problems

T lymphocytes, or T cells, are crucial for adaptive immunity, specifically in targeting intracellular pathogens. They develop in the thymus and migrate to secondary lymphoid organs. T cells can be activated by antigen-presenting cells, leading to differentiation into effector T cells, which respond immediately, or memory T cells, which provide long-term immunity. Cytotoxic T cells (CD8) induce apoptosis in infected cells, while helper T cells (CD4) activate other immune components. Understanding these functions is essential for grasping immune responses and the role of T cells in health and disease.

Introduction to T Lymphocytes

T lymphocytes, commonly known as T cells, are crucial components of the immune system, specifically involved in cell-mediated immunity, which is a subset of adaptive immunity. This type of immunity is essential for targeting and eliminating intracellular pathogens—disease-causing agents that invade host cells. T cells are primarily developed in the thymus, a primary lymphoid organ, and once matured, they migrate to secondary lymphoid organs such as the tonsils, lymph nodes, and spleen.

There are two main types of T cells: cytotoxic T cells (TC cells) and helper T cells (TH cells). Both types require activation by antigen-presenting cells, such as dendritic cells, which display antigens on their surface. Upon activation, T cells undergo division and differentiation into various forms, including memory T cells and effector T cells. Memory T cells, which can be either memory TC or memory TH cells, are essential for mounting a rapid response to subsequent infections by the same pathogen. In contrast, effector T cells, which include effector helper T cells and effector cytotoxic T cells, are responsible for executing immune functions against pathogens.

Understanding the roles and functions of T cells is vital for comprehending the broader mechanisms of the immune response. As we delve deeper into the study of T lymphocytes, we will explore their specific functions, activation processes, and interactions with other immune cells, including B cells, which will be discussed later in the course.

Which of the following answers is a major difference between cell-mediated and humoral immunity?

• A. Cell-mediated immunity utilizes T cells while humoral immunity utilizes B cells to respond to infection.
• B. Cell-mediated immunity involves antigens presented on APCs while humoral immunity involves free antigens.
• C. Cell-mediated immunity responds to antigens outside of cells. Humoral immunity responds to antigens within a cell.
• D. A and B.
• E. B and C.
• F. All of the above are major differences between cell-mediated and humoral immunity.

Answer: (D)

T-Cell Receptors

T cells are crucial components of the immune system, specifically involved in adaptive cell-mediated immunity. These cells develop in the thymus and possess thousands of identical T cell receptors (TCRs) embedded in their membranes. TCRs are specialized receptors that enable T cells to recognize and attack specific intracellular pathogens, which are pathogens residing within host cells. Importantly, TCRs do not bind to free antigens; they can only interact with antigens that are presented on the surface of antigen-presenting cells.

The structure of TCRs consists of two polypeptide chains: an alpha chain and a beta chain. These chains are linked by disulfide bonds, a type of covalent bond that stabilizes the structure. Each polypeptide chain has two distinct regions: a variable region and a constant region. The variable region varies among different TCRs and is responsible for binding to the presented antigen. This variation in the amino acid sequence of the variable region allows different TCRs to recognize a diverse array of antigens. However, each T cell expresses thousands of identical TCRs, meaning it can mount an immune response against only one specific antigen.

The constant region, in contrast, remains relatively unchanged in its amino acid sequence across different TCRs and is located closest to the cell surface. This stability is essential for the overall function of the TCRs. Understanding the structure and function of TCRs is fundamental to grasping how T cells identify and respond to pathogens, which will be further explored in subsequent lessons.

Which of the following is NOT a component of TCRs?

• A. Alpha chain.
• B. CD3 complex.
• C. Constant & Variable Regions.
• D. Beta chain.
• E. None of the above.

Answer: (B)

What is the importance of the variable region of T-cell receptors?

• A. The variable region is the closest region to the cell's surface and stabilizes the receptor in the plasma membrane.
• B. The variable region directly binds to antigens.
• C. The variable region varies in its amino acid sequence allowing different TCRs to bind different antigens.
• D. The variable region directly binds to antibodies.
• E. A and B.
• F. B and C.
• G. C and D.

Answer: (F)

Naive T Cells Become Effector & Memory T Cells

Naive T cells are a crucial component of the adaptive immune system, existing in an inactive state until they encounter a presented antigen. This activation typically occurs when a naive T cell interacts with an antigen-presenting cell, such as a dendritic cell. Upon activation, naive T cells can proliferate, creating identical clones, and differentiate into two main types of cells: effector T cells and memory T cells.

Effector T cells are short-lived and are responsible for generating an immediate immune response upon the first encounter with an antigen. In contrast, memory T cells are long-lived and can persist for many years, providing a rapid and robust response to subsequent infections by the same antigen. This memory function is vital for the effectiveness of vaccinations and natural immunity.

There are two primary subtypes of effector and memory T cells: cytotoxic T cells and helper T cells. Cytotoxic T cells, also known as CD8+ T cells, directly kill infected or cancerous cells. They can differentiate into effector cytotoxic T cells, which act quickly during an infection, or memory cytotoxic T cells, which remain in the body to respond more effectively to future infections. Helper T cells, on the other hand, assist in activating other immune cells, enhancing the overall immune response. They also differentiate into effector helper T cells and memory helper T cells, playing a pivotal role in orchestrating the immune response.

In summary, the journey from naive T cells to activated effector and memory T cells is essential for a robust immune defense. Understanding these processes helps clarify how the immune system adapts and remembers pathogens, ultimately leading to improved health outcomes.

A naive lymphocyte:

• A. Has encountered an antigen with its receptor but has not yet produced antibodies.
• B. Has not yet encountered an antigen recognized by its receptors but is producing antibodies.
• C. Has encountered an antigen and has undergone apoptosis.
• D. Has fully developed antigen receptors but has not yet encountered an antigen.
• E. Has produced antibodies despite not having developed receptors.

Answer: (D)

Which of the following statements about effector T cells is NOT true?

• A. Effector T cells are long-lived cells that allow the immune system to respond quickly to subsequent infections.
• B. Effector T cells can take two forms:Cytotoxic T Cells & Helper T Cells.
• C. Effector T cells trigger an immediate immune response to a novel infection.
• D. Effector T cells are short-lived immune cells.
• E. All of the above statements about effector T cells are true.

Answer: (A)

Cytotoxic T Cells vs. Helper T Cells

The immune system relies on two primary types of T cells, each with distinct roles in responding to antigens: cytotoxic T cells and helper T cells. Cytotoxic T cells, also known as CD8 cells, are characterized by the presence of CD8 proteins on their surface. Their main function is to identify and eliminate host cells infected with intracellular pathogens, such as viruses, by inducing apoptosis, a process of programmed cell death. This action is crucial for maintaining the integrity of the body’s defenses against infections.

In contrast, helper T cells, referred to as CD4 cells, possess CD4 proteins on their surface. These cells play a supportive role in the immune response by producing cytokines, which are signaling molecules that activate and stimulate other immune cells, including B cells. This activation is essential for orchestrating a robust immune response, enabling the body to effectively combat infections.

The differentiation between these two T cell types is facilitated by their respective CD markers, which are clusters of differentiation proteins that serve as identifiers. CD8 markers are exclusive to cytotoxic T cells, while CD4 markers are found on helper T cells. Understanding these distinctions is vital for comprehending how the immune system functions and how various immune cells interact to protect the body.

In summary, cytotoxic T cells are the body's defenders against infected cells, while helper T cells act as coordinators, enhancing the immune response by activating other immune components. This interplay between the two types of T cells is fundamental to the effectiveness of the adaptive immune response.

CD markers differentiate the two types of effector T cells. Cytotoxic T cells have _________ markers while helper T cells have _________ markers.

• A. CD3; CD4.
• B. CD4; CD5.
• C. CD8; CD6.
• D. CD8; CD4.

Answer: (D)

Which of the following statements about cytotoxic T cells is true?

• A. When cytotoxic T cells encounter an antigen, they produce cytokines to stimulate other immune cells.
• B. When cytotoxic T cells encounter “presented” antigens, they send signals triggering apoptosis in the infected cell.
• C. When cytotoxic T cells encounter an antigen, they recruit B cells to produce antibodies.
• D. Cytotoxic t cells only recognize and respond to “free” antigens.

Answer: (B)

Here’s what students ask on this topic:

What are the main functions of cytotoxic T cells (CD8) and helper T cells (CD4)?

Cytotoxic T cells (CD8) and helper T cells (CD4) have distinct roles in the immune response. Cytotoxic T cells, also known as CD8 cells, are primarily responsible for inducing apoptosis in host cells infected with intracellular pathogens, such as viruses. They achieve this by releasing chemicals that trigger cell death, thereby eliminating the infected cells. On the other hand, helper T cells, or CD4 cells, do not directly kill infected cells. Instead, they produce cytokines that help activate and stimulate other immune cells, such as B cells and macrophages, enhancing the overall immune response. This activation is crucial for a coordinated and effective defense against pathogens.

How do T cell receptors (TCRs) recognize antigens?

T cell receptors (TCRs) recognize antigens through a highly specific interaction with antigen-presenting cells (APCs). Each TCR is composed of two polypeptide chains, an alpha chain and a beta chain, linked by disulfide bonds. These chains have variable regions that differ between TCRs, allowing them to bind to a wide variety of antigens. However, TCRs can only bind to antigens that are presented on the surface of APCs, such as dendritic cells, in the context of major histocompatibility complex (MHC) molecules. This specificity ensures that T cells are activated only in the presence of a specific antigen, leading to a targeted immune response.

What is the difference between naive, effector, and memory T cells?

Naive T cells are inactive T cells that have not yet encountered their specific antigen. Upon encountering an antigen presented by an antigen-presenting cell, naive T cells become activated and differentiate into effector and memory T cells. Effector T cells are short-lived and provide an immediate immune response to the current infection. They include cytotoxic T cells that kill infected cells and helper T cells that activate other immune cells. Memory T cells, on the other hand, are long-lived and persist in the body for years, providing a rapid and robust response to subsequent exposures to the same antigen, thereby offering long-term immunity.

Where do T cells develop and where do they migrate after development?

T cells develop in the thymus, a primary lymphoid organ. Once they are fully developed, they migrate to secondary lymphoid organs, such as the tonsils, lymph nodes, and spleen. In these secondary lymphoid organs, T cells encounter antigen-presenting cells that present specific antigens, leading to their activation. This migration and subsequent activation are crucial for the T cells to perform their roles in adaptive immunity, targeting and destroying intracellular pathogens and coordinating the immune response.

How do memory T cells contribute to long-term immunity?

Memory T cells contribute to long-term immunity by persisting in the body for many years, sometimes even the lifetime of the individual. These cells are generated after the initial activation of naive T cells during the first encounter with a specific antigen. Unlike effector T cells, memory T cells do not respond to the initial infection but are primed to respond rapidly and robustly to subsequent exposures to the same antigen. This quick response helps to prevent reinfection and provides a stronger and more efficient immune defense upon re-exposure to the pathogen.