Difference Between Immune Response to Bacteria and Virus

The main difference between immune response to bacteria and virus is that the immune system attacks bacteria by complement proteins and phagocytosis whereas the immune system recognizes virus-infected cells through epitopes presented by MHC molecules. Generally, bacteria live outside the host cells; thus, components of the immune system can easily attack them. In contrast, viruses live inside the host cells and therefore, immune system components are unable to recognize them. 

Bacteria and virus are the two types of pathogens that can invade host organisms. The immune system can respond to both types of pathogens in different ways by triggering an immune response. 

Key Areas Covered 

1. What is an Immune Response to Bacteria
     – Definition, Facts, Methods
2. What is an Immune Response to Virus
     – Definition, Facts, Methods
3. What are the Similarities Between Immune Response to Bacteria and Virus
     – Outline of Common Features
4. What is the Difference Between Immune Response to Bacteria and Virus
     – Comparison of Key Differences

Key Terms

Complement System, Immune Response to Bacteria, Immune Response to Virus, Interferon, Phagocytosis 

What is the Immune Response to Bacteria 

The immune response to bacteria is a response to intracellular pathogens since bacteria live outside the cells. Several types of immune responses are responsible for the removal of bacteria from the body. They are complement-mediated lysis, phagocytosis, and adaptive immunity. 

Complement-Mediated Lysis 

A type of protein in the immune system called complement protein attacks bacteria and facilitates the killing of bacteria by three methods. The first method is the classic complement pathway initiated by the binding of antibodies to bacteria. The C1 complement protein complex binds to the tails of the antibodies, initiating a cascade of cleavage. Also, C1 reforms complement complexes on the surface of the bacteria in the form of membrane attack complex (MAC) or by means of opsonins. This induces the osmotic lysis of bacteria by producing pores on their surface. 

The second is the alternative complement pathway in which the C3 complement proteins bind to bacteria, activating the complement cascade that destroys bacteria through MAC. 

The third is the lectin pathway in which mannose-binding lectin (MBL) bind to the mannose residues in some bacteria. This sequentially activates C2 and C4 complement proteins to destroy bacteria. 

Phagocytosis 

Phagocytes recognize opsonized bacteria, engulfing and digesting them by phagocytosis. Some phagocytes are macrophages, dendritic cells, neutrophils, monocytes, and mast cells.

Figure 1: Activation of Phagocytes

Adaptive Immunity 

Once digested, epitopes are presented to the circulating helper T cells by the MHC class II molecules on the cell membrane of phagocytes. The Th1 helper T cells produce interferon-g (INF-g) that triggers the cell-mediated immune response while Th2 helper T cells produce interleukin-4 that trigger a humoral immune response by recognizing the epitopes. 

What is the Immune Response to Virus 

An immune response to a virus is a response to intercellular pathogens since viruses live inside the host cells, completing its life cycle. The methods immune system use to respond to virus are via cytotoxic T cells, interferons, and antibodies.  

Cytotoxic T Cells 

The virus-infected cells present viral epitopes on their cell membrane through MHC class I molecules. The cytotoxic T cells specifically recognize these epitopes through their T cell receptors (TCRs) on the cell membrane and release cytotoxic factors that kill the virus-infected cells. 

However, some viruses can prevent their epitope presentation by reducing the expression of MHC class I molecules on the cell membrane. But the natural killer cells recognize such cells with reduced MHC class I molecules on the cell membrane and kill them, suspecting them as virus-infected cells.

Figure 2: Natural Killer Cells Function

Interferons 

Virus-infected cells secrete interferons that prevent the replication of the virus. Interferons also signal nearby cells to increase their MHC class I molecules on the cell membrane. 

Antibodies 

Before entering into a cell, antibodies can catch viruses. Many antibodies form an agglutination of antibodies that can activate phagocytosis of the virus. Also, antibodies can activate the complement system to destroy the virus. 

Similarities Between Immune Response to Bacteria and Virus 

• Both innate and adaptive immunity trigger immune responses to bacteria and virus. 
• Antibodies are involved in both types of immune responses. 

Difference Between Immune Response to Bacteria and Virus 

Definition 

Immune response to bacteria refers to an immune response to intracellular pathogens while the immune response to the virus refers to an immune response to intercellular pathogens. 

Theory 

The immune system recognizes the viral infected cells through the viral epitopes presented by MHC molecules while the immune system can recognize and destroy bacteria by phagocytosis and complement system. This is the main difference between immune response to bacteria and virus.

Responding Methods 

The responding methods to bacteria are complement-mediated lysis, phagocytosis, and adaptive immunity while the responding methods to the virus are cytotoxic T cells, interferons, and antibodies. 

Conclusion 

Immune response to bacteria occurs through the innate immune responses such as complement system and phagocytosis. Generally, bacteria are intracellular pathogens. The immune response to virus occurs mainly through adaptive immunity such as cytotoxic T cells and antibodies. Viruses are intercellular pathogens. The main difference between immune response to bacteria and virus is the type of immune response. 

Reference:

1. Laing, Kerry. “Immune Responses to Bacteria.” British Society for Immunology, Available Here
2. Laing, Kerry. “Immune Responses to Viruses.” British Society for Immunology, Available Here

Image Courtesy:

1. “Commensals vs pathogens mechanism” By БИОлогиня – Own work (CC BY-SA 3.0) via Commons Wikimedia   
2. “Figure 42 02 07” By CNX OpenStax – http://cnx.org/contents/[email protected]:rZudN6XP@2/Introduction (CC BY 4.0) via Commons Wikimedia