What is the Difference Between CD4 and CD8 T Cells

The main difference between CD4 and CD8 T cells is that the CD4 T cells are the helper T cells, which assist other blood cells to produce an immune response, whereas the CD8 T cells are the cytotoxic T cells that induce cell death either by lysis or apoptosis. 

CD4 and CD8 T cells are two types of T lymphocytes mainly involved in the cell-mediated immunity. Furthermore, CD4 T cells produce cytokines to activate immune cells including B cells, CD8 T cells, and macrophages while CD8 T cells destroy virus-infected cells and tumour cells. 

Key Areas Covered 

1. What are CD4 T Cells
     – Definition, T Cell Receptor, Immune Response
2. What are CD8 T Cells
     – Definition, T Cell Receptor, Immune Response
3. What are the Similarities Between CD4 and CD8 T Cells
     – Outline of Common Features
4. What is the Difference Between CD4 and CD8 T Cells
     – Comparison of Key Differences

Key Terms 

CD4 T Cells, CD8 T Cells, Cell Death, Cell-Mediated Immunity, Cytokines, T Cell Receptor, T Lymphocytes

What are CD4 T Cells 

CD4 T cells are the helper T cells (TH cells) that express CD4 glycoprotein on the cell membrane as the T cell receptor. They are responsible for the activation or suppression of the function of the other cells in the immune system. This mediation occurs through various types of cytokines secreted by CD4 T cells. Antigen-presenting cells including macrophages, neutrophils, and dendritic cells take up extracellular pathogens including bacteria and viruses to destroy them while processing their antigens. These processed antigens are presented on the cell membrane of the antigen-presenting cells along with MHC class II molecules. The CD4 T cells recognize these antigens through their T cell receptors and secrete cytokines.  

Figure 1: T Cell Activation

These cytokines include interleukins and IFN- γ. However, the type of cytokines produced depend on the type of CD4 T cells. The main types of CD4 T cells are TH1, TH2, TH17, and TFH. The effector cells of the CD4 T cells are B cells, CD8 T cells, and macrophages. Cytokines are responsible for the maturation of B cells into plasma cells and memory B cells. In addition, CD8 T cells mediate cytotoxicity while macrophages destroy pathogens via phagocytosis. 

What are CD8 T Cells 

CD8 T cells are the cytotoxic T cells (TC cells) or killer T cells that express CD8 glycoprotein on the cell membrane as their T cell receptor. The main function of cytotoxic T cells is to induce cell death in virus-infected cells and tumor cells either through cell lysis by degranulation or apoptosis. Here, all nucleated cells in the body can present antigens to the CD8 T cells along with the MHC class I molecules. For example, any virus-infected cell can present viral antigens to the CD8 T cells. Upon recognition, CD8 T cells induce cell death of that infected cell. In addition, the cytokines produced by the CD4 T cells stimulate CD8 T cells. 

Figure 2: CD4 and CD8 T Cell Function

CD8 T cells send proteases and other enzymes to the infected cells through a microtubular cytoskeleton. On the other hand, some cytokines including IL-10 secreted by another type of T cells called regulatory T cells, inactivate CD8 T cells to an anergic state in order to prevent the recognition of self-antigens as non-self. This reduces autoimmune diseases. 

Similarities Between CD4 and CD8 T Cells 

• CD4 and CD8 T cells are two types of T lymphocytes in the blood. 
• Both contain T cell receptors whose variation helps to identify the type of T cells. 
• Also, both differentiate from the common lymphoid progenitor in the bone marrow and mature in the thymus. 
• Moreover, they are involved in triggering a cell-mediated immune response. 

Difference Between CD4 and CD8 T Cells 

Definition 

CD4 T cells refer to T cells that express CD4 glycoprotein on the cell membrane. CD8 T cells refer to T cells that express CD8 glycoprotein on the cell membrane. Hence, these explais the funadamental difference between CD4 and CD8 T cells.

Also Known as 

CD4 T cells are also known as helper T cells while the CD8 T cells are also known as cytotoxic T cells. 

Type of T Cell Receptor 

The CD4 glycoprotein serves as the T cell receptor in CD4 T cells while the CD8 glycoprotein serves as the T cell receptor in the CD8 T cells. Therefore, this is also a difference between CD4 and CD8 T cells.

Antigen Presentation 

Another difference between CD4 and CD8 T cells is that the CD4 T cells recognize antigens on the surface of the antigen-presenting cells while the CD8 T cells recognize antigens on the surface of all nucleated cells. 

MHC Complex 

CD4 T cells recognize antigens presented along with MHC class II molecules while CD8 T cells recognize antigens presented along with the MHC class I molecules. This is yet another difference between CD4 and CD8 T cells.

Role 

There is also a difference between CD4 and CD8 T cells based on their function and role. CD4 T cells induce other immune cells to trigger an immune response by secreting cytokines upon recognizing antigens while CD8 T cells induce cell death of the virus-infected cells or tumour cells either by cell lysis or apoptosis. 

Conclusion 

CD4 T cells are helper T cells with CD4 glycoprotein on their cell surface as the T cell receptor. They recognize antigens presented along with the MHC class II molecules by antigen-presenting cells. Then, they secrete cytokines to induce other cells in the immune system including B cells, CD8 T cells, and macrophages to trigger an immune response. On the other hand, CD8 T cells are the cytotoxic T cells which express CD8 glycoprotein as the T cell receptor. They recognize antigens presented along with the MHC class I molecules on the surface of all nucleated cells and are responsible for the cell death in virus-infected cells and tumour cells. Therefore, the main difference between CD4 and CD8 T cells is the type of T cell receptor on the cell membrane and the type of immune response they generate. 

Reference:

1. Janeway CA Jr, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001. Chapter 8, T Cell-Mediated Immunity. Available Here
2. Janeway CA Jr, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001. T cell-mediated cytotoxicity. Available Here.

Image Courtesy:

1. “T cell activation” By T_cell_activation.png: Template drawing and caption text from “The Immune System”, any modifcations, made by myself are released into the public domain.derivative work: Hazmat2 (talk) – This file was derived from: T cell activation.png (Public Domain) via Commons Wikimedia
2. “Figure 42 02 04” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia