Difference Between CD4 Cells and CD8 Cells

Key Difference – CD4 Cells vs CD8 Cells
 

In the context of cell-mediated immunity, T cells, generally referred to T lymphocytes, play an important role. Since they mature in the thymus from thymocytes, they are referred to as T cells. T cells have two main categories: T helper (Th) cells and cytotoxic T cells (Tc).  Due to the presence of two different types of glycoproteins, i.e., CD4 and CD8, on the cell surface of Th cells and Tc cells, they are referred to as CD4+ T cells and CD8+ T cells, respectively. CD4+ T cells recognize antigens presented by Major Histocompatibility Complex (MHC) Class II and activate in order to kill intracellular microorganisms by releasing cytokines. CD8+ T cells only recognize antigens presented by MHC Class I and destroy infecting tumor cells and viruses directly.  This is the key difference between CD4 cells and CD8 cells.

CONTENTS

1. Overview and Key Difference
2. What are CD4 Cells 
3. What are CD8 Cells
4. Similarities Between CD4 Cells and CD8 Cells
5. Side by Side Comparison – CD4 Cells vs CD8 Cells in Tabular Form
6. Summary

What are CD4 Cells?

CD4 is considered as a glycoprotein which plays a major role in the immune system. CD4 lies on the surfaces of some immune cells like dendritic cells, T helper cells, macrophages, and monocytes. CD4 protein is usually encoded by a gene called CD4 gene in humans. CD4 possess a short cytoplasmic tail which consists of a special amino acid sequence that helps to initiate and communicate with tyrosine kinase Lck. This Lck is needed to activate the molecular components of the signaling cascade of a T cell which is activated. CD4 belongs to the immunoglobulin superfamily just as other cell surface receptors. It consists of four immunoglobulin domains D₁ to D₄.  These domains are situated on the extracellular surface of the respective cells. D₁ and D₃ are similar to immunoglobulin variable (IgV) domains while D₂ and D₄ are similar to the immunoglobulin constant (IgC) domains. CD4 interacts with β₂-domain of major histocompatibility complex (MHC) class II molecules with the help of D₁ domain. Therefore, these CD4 become specific for antigens which are presented only by MHC class II.

Figure 01: CD4 Cells

CD4 is known as a co-receptor of the T cell receptor (TCR). This helps in the communication with the antigen- presenting cells. CD4 and the TCR complex each bind to specific regions of the antigen-presenting cells with the influence of the extracellular D₁ domain. There are diseases which are caused due to defects of CD4. For example, in HIV infection, the HIV-1 virus enters the host T-cells through CD4, and the number of T cells which express CD4 are also subjected to a progressive reduction.

What are CD8 Cells?

CD8 is considered as a transmembrane glycoprotein which functions in the immune system. CD8 is also known as a co-receptor of the T cell receptor (TCR). Similar to the TCR, CD8 binds to the major histocompatibility complex (MHC) class I protein specifically. The CD8 are mainly located on the surface of cytotoxic T cells and cortical thymocytes, natural killer cells, and dendritic cells. Just like CD4, CD8 also belongs to the immunoglobulin superfamily. In order to facilitate the function, the CD8 forms a dimer which consists of a CD8 chain pair. The common types of CD8 are CD8-α and CD8-β.  It consists of an immunoglobulin variable (IgV) like extracellular domain connecting to the membrane by a stalk and an intracellular tail. Normally, the IgV, like extracellular domain of the type CD8-α, cooperates with the class I MHC molecules.  This affinity between the molecules keeps the cytotoxic T cell’s T cell receptor tightly bound together with the target cell during the activation of antigen specificity.

Figure 02: CD8 Cells

What are the Similarities Between CD4 Cells and CD8 Cells?

• CD4 and CD8 are surface proteins which are found on the surface of their respective cells.

• Both CD4 and CD8 are generated in the thymus and express the T-cell receptor.
• Both of them are considered as glycoproteins and belong to the immunoglobulin superfamily.

• Both are able to bind to MHC molecules in the absence of T cell receptor. CD4 and CD8 are also capable of improving the antigen-induced IL-2 production with different mechanisms.

What is the Difference Between CD4 Cells and CD8 Cells?

Summary – CD4 Cells vs CD8 Cells

T cells are important in cell-mediated immunity. They are matured in the thymus from thymocytes. T cells are distinguished from other lymphocytes due to the presence of T cell receptor. T cells are of two types: Th cells and Tc cells. The glycoproteins CD4 and CD8 are present on Th cells and Tc cells respectively. CD4+ T cells recognize antigens presented by Major Histocompatibility Complex (MHC) Class II and activate in order to kill intracellular microorganisms by releasing cytokines. CD8+ T cells only recognize antigens presented by MHC Class I and destroy infecting tumor cells and viruses directly. This is the difference between CD4 cells and CD8 cells.

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Reference:

1. “Adaptive immunity.” Khan Academy, Available here. Accessed 18 Sept. 2017.
2. “CD8 T Cells.” British Society for Immunology, Available here. Accessed 18 Sept. 2017.

Image Courtesy:

1. “Antigen presentation” By user: Sjef – self made, referring to this image. (CC BY-SA 3.0) via Commons Wikimedia

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