The Human Body’s Reaction to Viruses

Day in and day out, the human body interacts with microorganisms beneficial or detrimental to our health. Every day our immune system protects us from these threats before they can cause any damage to our body. The COVID-19 pandemic has awakened opinions and research on the best defense against this virus. An important part in analyzing these statistics and commentary relies on understanding how our bodies react to viruses. The following information explains our immune system’s processes to fighting infections.

​The Basics
Innate immune response builds the first line of defense against pathogens (disease-causing microorganisms like viruses or bacteria). The innate immune system is the first to react to a pathogen invasion, can identify patterns commonly familiar with pathogens as opposed to distinct ones, and is always consistent with its response to these threats. The system is a combination of cells, barrier tissues, and molecules. Barrier tissues are located where our bodies interact with the environment. Cells in innate immunity are various types of white blood cells and cells that live in the tissues. There are receptors on these cells that bind to threats and recognize pathogen-associated molecular patterns (PAMPs). Examples of PAMPs are viral DNA and RNA. After recognition, the cells react by ingesting and killing the pathogen, release molecules to recruit more cells to the site, and/or kill other cells stressed or infected by the virus. The larger issue in the whole process is that while our body is controlling the infection, the resulting inflammation can cause organ damage if the virus created an intense response.

​Microbes and Innate Immunity
A microbial infection (i.e bacterial infection) is an example of how the innate immune system alters its response based on the pathogen type. The cells consistently present in the tissues are called sentinel cells, and they are the first cells to respond and react to an incoming pathogen. Once the sentinel cells notice the PAMPs, they release pro-inflammatory mediators/molecules to encourage blood vessels to relax and ease the connections between cells to permit fluid and molecule movement into the tissue. Complimentary proteins flowing through the blood enter the infected tissue to bind and kill the pathogens. Cytokines (signaling molecules) also take their part in stimulating blood vessels for more circulating immune cells to stick to the infection by way of adhesion molecules located on the surface of blood vessel endothelial cells. The incoming immune cells like white blood cells enter the tissue to ingest and kill the pathogens before undergoing cell death. Monocytes go under the same procedure, but also eat the remains of the dead cells. Homeostasis is restored when all the pathogens and dead cells are cleared from the area. Proteins made by those pathogens are reused and transformed into peptides. These peptides are carried into lymphatic vessels to lymph nodes for the T cells in the adaptive immune system for future similar infections.

​The Adaptive Immune System
The innate immune system responds to a general threat, but it has restraints on how it encounters specific threats. The adaptive immune system’s response acts powerfully on specific and frequent dangers. Differing from the innate immune system, this system customizes its response to certain antigens, is aware of which invading molecules are harmless or dangerous, and remembers those distinct threats and quickly targets them. Cells inside are lymphocytes, and they are broken down into two categories. T cells acknowledge the peptides, kill the pathogens, and assist other immune cells. B cells create antibodies to help remove pathogens. Antibodies are proteins that bind with antigens. All antibodies form the same structure, but function differently based on their isotypes. Some isotypes can perform neutralization so they can bind with antigens to protect other molecules. Compliment fixation is another function where the antibody binds complementary proteins to a pathogen’s surface causing cell rupture. Opsonization is the method of covering molecules on a pathogen so it is branded as indigestible by the immune system. The adaptive immune system also has NK cells (Natural Killer cells) that execute cell death of infected cells that have been detected by antibodies.

​How B Cells Respond
B cells produce antibodies. Stimulating the creation of an antibody library how vaccines protect our bodies against viruses. B cells originate from bone marrow and have a unique cell receptor that distinguishes specific antigens. During their circulation in secondary lymphoid organs, they are activated when discovering a protein antigen and internalizes it. The B cell administers processed peptides from the protein and lets them interact with an activated T cell. With assistance from the T cell, the B cell endures a cloning process to produce other B cells with receptors specific to that antigen type. The activated B cells differentiate into plasma cells with matching antigen binding receptors similar to the original B cell. At germinal centers, separating B cells undergo isotype switching (changing receptors so divergent isotypes can be created and make a better immune response) and affinity maturation (receptors mutate and the best affinity B cells are chosen for memory cells and long-lived plasma cells). After the first B cell response to the antigen, the new antibodies and memory now react faster and more efficiently to that antigen if it ever appears again.

​The T Cells
T cells were previously mentioned as one of the cells in the adaptive immune system. These cells come from bone marrow and relocate to the thymus to continue their development. During their maturity phases, the T cells’ receptors are attracted peptides from an antigen once it has infected a cell. In other words, it skips the antibody process of the B cells and goes directly to the infected cells. Every receptor can distinguish varying peptides as long as they are attached to certain molecules. When they fully mature and are not activated by an antigen, they circulate around the body in the secondary lymphoid organs.

​How T Cells Respond
In the event of a viral invasion and after the innate immune response, the viral peptides on the molecules of dendritic cells entering the lymph nodes are confronted by the unactivated T cells. The T cells are only activated when they are given two signals during binding. The first signal is the initial binding of the peptide-molecule complex to the cell. The second signal is confirmation from the dendritic cell to complete the binding. Now activated, the T cells will proceed with clonal expansion to alter into effector T cells and most of those cells are assigned to that one antigen. These cells are allowed to exit the lymph nodes to execute immune operations. They travel throughout the body and destroy any cell containing the specified the complex recognized by the initial activated T cell. Effector cells have other functions including killing tumor cells from mutated proteins and helping other immune cells. After an infection is terminated, long-lived T cells will stay in the human body in case of future reactivation by the same invading pathogen. Thus, the process begins again but more robust than previously.