How are we protected from virus infection?

Viruses can cause havoc in the animal organism, taking over the cellular machinery, destroying cells that line the capillaries and producing massive haemorrhages, affecting vital organs such as the liver or the kidneys. Their weapons are simple but effective. Do we animals have weapons to defend ourselves?

Obviously yes, because if not, we would not be here: we would have become extinct. We could spend a whole course studying the mechanisms of defence, but we will have to limit ourselves to describe very briefly what they are and how they work. To begin with, there are innate and acquired mechanisms. Innate immunity is the first line of defence against viruses and acts continuously, producing an immediate response. However, there is no memory or ability to respond more intensely in second encounters with the virus. Also, it is unable to distinguish subtle differences among the molecules. However, it allows time for the acquired response to develop. Acquired immunity is self-limiting and disappears when the stimulus disappears.

It has memory and is specific, being capable of differentiating small differences between many molecules, that we will call antigens. Remember these features because they will be essential to understand how vaccines work. Both cells and molecules are involved in innate and acquired immunity. Cells of the innate response are mainly in charge of destroying pathogenic agents by phagocytosis. Some of these cells are capable of processing these pathogens and presenting their antigens on their surface to activate the acquired response. There are many molecules of innate immunity. Alpha interferon, a molecule produced by cells infected by viruses, protects neighbouring cells from infection. Furthermore, complement can produce lysis of the virus, of the infected cell and attract more cells to fight.

Acquired immunity has two main branches: the humoral immunity or that mediated by antibodies and the cell-mediated immunity. The first is in charge of B cells and plasma cells, while the second is in charge of cytolytic and helper T-lymphocytes or Th. The molecules they secrete are antibodies and other various types, classified as cytokines. Antibodies are effective mainly against extracellular viruses, while cell-mediated immunity is more effective against virus-infected cells. To eliminate extracellular virus, antibodies can activate complement, lysing virus, or can attach to it and draw it towards phagocytic cells as in an ambush, a process known as opsonization.

Antibodies can also form a net around several viruses, agglutinating them and avoiding their spread, or block their entry into the cell that they want to infect, a process known as neutralization. Remember that antibodies are ALWAYS specific for that particular virus. The acquired immunity is also in charge of removing cells infected by the virus, so that they do not continue producing more virions. There are also different mechanisms, some of them mediated by antibodies, such as complement activation, and antibody-dependent cellular cytotoxicity also called ADCC. The cytolytic T-lymphocytes and natural killer cells secrete cytokines. The result of all this is the lysis of the infected cell. Usually viral infection produces an acute initial process, with a multitude of clinical signs.

After that, the immune system is usually able to remove the virus and stop the progress. But on other occasions the virus multiplies constantly in the infected cell without killing it, a process that is known as persistent infection. There are different types of persistent infections. Some viruses, as the herpesvirus, can set latent or silent infections in certain tissues of the host, a process characterized by the absence of production of infective virus during long periods. In the chronic infections there is a constant presence of infective virus, even in the absence of clinical signs. This is dangerous, because the animal, known as the carrier, may continue transmitting the infection.

On other occasions, the genome of the virus, especially retroviruses, is inserted into the genome of the cell, becoming expressed only when there are adequate stimuli. Finally, some viruses are capable of inducing the cell to multiply disorderly, inducing cancer. These are the transforming or oncogenic infections. We have learned many things in this activity. We have started with the defence mechanisms of the animals, the innate immunity that is always there, allowing time for acquired immunity to be formed; and how, if the defence mechanisms fail, viral infection can develop. All this will help you understand the activities of the weeks to follow.