Immune Response to Virus vs Bacteria

Case Study: A group of your colleagues have booked a cruise for ten days and want you to come with them. You ask them if they’ve seen the movie Titanic. You give it some thought. It’s the end of February, and you’re tired of the winter, dry chapped skin, and not being able to wear fashionable footwear. You refill your anti-anxiety medication and agree to join them. Two days into the cruise you’re feeling a bit nauseous. You chalk it up to seasickness and hurl over the side of the boat which your colleagues find amusing. So much for the Dramamine and your scopolamine patch. You quickly realize that your hyperactive bowel sounds, abdominal pain, and distended abdomen are not consistent with being seasick. Luckily you make it back to your room before having a lower GI eruption. For the next 12 hours, you vomit and have diarrhea every 15 minutes. You’re fairly certain you’re going to die. You want to die. You stumble to the infirmary which is full of other people that are sick just like you. Having just stopped in Mexico and visited Aztec ruins, you offer several of your colleagues as human sacrifices in exchange for IV fluids and Zofran (an antiemetic). The nurse kindly tells you that won’t be necessary. After receiving four liters of IV fluid and IV Zofran you decide that you’re not walking toward the light today. Although your nausea, vomiting, and diarrhea have subsided you have a low-grade fever, headache, and joint and muscle pain. When you go back to your room you see in the news that your cruise ship will be docking in Miami because more than 400 people on your cruise ship have Norovirus. _________________________________________________________________ Describe the normal immune response to a viral infection? To guard against infections, the body has in place several mechanical barriers such as the skin, mucosal surfaces, and the hostile environment of the gut. Viruses are usually contracted from person-to-person contact, but as seen in this case study, can also be contracted by ingesting contaminated food and water as well. The common routes of entry or infection include eyes, respiratory tract, skin, genitourinary tract, and gastrointestinal tract.1 Infections caused by viruses and bacteria are both spread in the same ways. Viruses are unique in that they aren’t actually living organisms. They are noncellular particles containing genetic material that rely on the replication of a host cell to survive. Because of this mechanism of viruses to hijack and replicate through host cells, detection and response to them by our immune system is rather unique. Since they essentially hide inside cells, the immune system has its own system of inspecting the contents of each other's cells to detect potential foreign viral invaders hiding inside them. There are three main ways in which the immune system responds to viral infection--through cytotoxic cells, interferons, and antibodies. Cells contain class 1 major histocompatibility complex proteins ( or MHC class 1 proteins) that work by displaying pieces of protein from inside the cell upon the cell surface--including proteins made by viruses.2 They become markers for cell contents. A type of T cell called the cytotoxic T cell has specialized T cell receptor (TCR) proteins on its surface that recognizes virally infected cells. If it detects antigenic peptides bound to MHC molecules on another cell’s surface--an infected cell--then the T cell responds by killing that cell to prevent spread of the virus.2 Some viruses however have ways of avoiding detection. One of these being to stop MHC molecules from displaying viral peptides on the cell surface.2 In this case, the T cell cannot detect viral presence in that cell. These cells with less MHC molecules displayed become targets for another cell in our immune system, called the natural killer cell (NK cell).2 If found by an NK cell, it will release toxic substances that kill the infected cell. Cytotoxic factors stored in granules of cytotoxic T cells and NK cells are released when triggered.2 Perforin is one mediator that creates pores in the membranes of target cells.2 Granzymes are another mediator that enters these pores, and initiate in the target cell the sequence of apoptosis (a pre-programmed cell death, or self-destruct button, in a way). Granulysin is a mediator that uses lysis to attack and destroy the target cell’s outer membrane.2 Proteins called cytokines are released when triggered by the target cell, as well. These include interferon-g and tumour necrosis factor-a, which work transferring signals between the T cell and surrounding cells to enhance the killing mechanisms.2 Interferons are produced by the virally infected cell. They work by interfering with the virus's replication from within the cell; signalling nearby cells of viral presence, which then triggers those cells to display more MHC class 1 molecules on their surface thereby increasing the ability to detect infected cells in the area. Antibodies are proteins that detect and bind to pathogens before they enter a host cell. Antibodies neutralize the virus, work with other activated antibodies by binding particles together in a process called agglutination, and attach viruses to Fc receptors on phagocytic cells which then phagocyte (eat and destroy) the virus.2 Antibodies also trigger what is called the complement system, or complement cascade which opsonises and promotes further phagocytosis of viruses.2 How does the immune response to a viral infection differ from the response to a bacterial infection? The difference between the immune system’s response to bacterial versus viral infections is that bacteria are primarily attacked via complement proteins (the complement and alternative pathways), whereas viruses are attacked via cytotoxic T cells, interferons, and antibodies. People often refer to viral gastroenteritis as the “stomach flu” which drives Dr. Rosebush insane. Why is this incorrect? The problem with giving gastroenteritis the nickname of “stomach flu” is that actual flus (influenzas) are viruses that affect the respiratory system.3 This isn’t an aptly chosen nickname for gastroenteritis, which as the “gastro” in its name tells us, is a viral infection that affects the gastrointestinal organs (digestive system). What happens to your WBC (white blood cell count) when you have a viral infection as opposed to a bacterial infection? In bacterial infections, neutrophils increase while lymphocytes decrease. In viral infections, neutrophils decrease while lymphocytes increase.4 It is notable to mention that viral infections can lead to temporary leukopenia (decrease WBC count), due to a disruption of the production of WBCs in the bone marrow. References Mueller SN, Rouse BT. Immune responses to viruses. Clinical Immunology. 2008;421-431. doi:10.1016/B978-0-323-04404-2.10027-2 Immune responses to viruses. Immunology.org. Accessed April 11, 2021. https://www.immunology.org/public-information/bitesized-immunology/pathogens-and-disease/immune-responses-viruses Viral gastroenteritis (stomach flu). Mayoclinic.org. Accessed April 11, 2021. https://www.mayoclinic.org/diseases-conditions/viral-gastroenteritis/symptoms-causes/syc-20378847 Complete blood count with differential. Rnceus.com. Accessed April 11, 2021. https://www.rnceus.com/cbc/cbcdiff.html