Vaccine Immunology
The Y-shaped structure of antibodies has two variable antigen-binding fragments at the ends, which recognize and bind to specific antigens. Image courtesy of the National Institutes of Health and National Human Genome Research Institute.
1) Vaccination introduces an antigen, which is displayed by antigen-presenting cells. Binding of the antigen will activate cells in the immune system, including memory cells that will recognize the antigen in the future.
2) When the actual pathogen enters the body, the memory cells recognize the antigens on the pathogen's surface and will respond quickly to neutralize it.
Image courtesy of Harvard University: Graduate School of Arts and Sciences.
When a foreign substance such as a virus, bacteria, or other type of microorganism, invades the body, the innate and adaptive immune systems unleash a series of steps to attack and neutralize the invading pathogen. The innate immune system is the first line of defense against invading pathogens. It encompasses both simple physical defense mechanisms, such as mucous membranes that trap foreign invaders, as well as cellular defense mechanisms, such as phagocytosis and immune activation by pattern recognition receptors (PRRs). Pattern recognition receptors are predominately expressed on immune cells and recognize a wide variety of conserved pathogen-associated molecular patterns (PAMPs), specific to the invading microorganism. When pattern recognition receptors bind to these molecular patterns, called antigens, mostly located on a pathogen’s surface, it activates the release of monocytes and macrophages. Macrophages and monocytes are types of white blood cells responsible for ingesting the pathogen and destroying it. Macrophages can also digest the pathogen and expose the antigen structures on their surface in order to activate cells of the adaptive immune system. Given that the innate immune system does not recognize specific microorganisms, but instead identifies the common molecular patterns found on these microorganisms, it does not confer long-term immunity against a particular pathogen.
Unlike the innate immune system, the adaptive immune system is responsible for recognizing specific pathogens and includes cells that allow for long-term immunity against these pathogens. The most important type of cells in the adaptive immune system are the B- and T-lymphocytes. Immature B-lymphocytes, or B-cells, are produced in the bone marrow. When an antigen is introduced into the body, via natural pathways or vaccination, this antigen will bind to the antigen-binding region (Fab) on a B-cell receptor. Once bound to an antigen and activated by T-helper cells, the B-cell will alter the antigen-binding region to achieve the best fit between the antigen and Fab regions. Activation will then stimulate the B-lymphocytes to mature into plasma cells responsible for producing antibodies with the best recognition site to bind the antigen. Antibodies are traditionally depicted as Y-shaped structures with variable antigen-binding regions (Fab) at the ends, which recognize and bind to a specific antigen.
Once an antigen is bound to an antibody, it is marked for destruction by the body. The first type of antibodies to be released are IgM antibodies, which are specific to the antigen and help eliminate the pathogen from the body. As the immune response progresses, IgG antibodies will be produced, which bind the antigen more effectively and better neutralize the pathogen. For this reason, the production of IgG antibodies is more important for the success of a vaccine. T-lymphocytes require a different activation mechanism where pathogens are digested, and the corresponding antigen is expressed on the surface of antigen-presenting cells, such as macrophages and dendritic cells. When the T-lymphocyte binds to the antigen on the antigen-presenting cell, it becomes activated and will produce more T-cells. Activated B- and T-lymphocytes will also produce memory cells. Both types of memory cells circulate in the body long after the first immune response and “remember” if an antigen has previously infected the body. Upon re-infection, the memory cells will allow the onset of the immune response to be faster and more effective. The simultaneous activation of immature B-lymphocytes through binding of an antigen and stimulation from T-helper cells, produces the strongest memory response of all immune pathways. It is, therefore, a desired outcome of vaccines.
What is Herd Immunity and Why is it Important?
Herd immunity takes place when a large percentage of a population is immune to a particular disease. Herd immunity can be achieved by building immunity either through vaccination or recovery from the disease. When few to no individuals have immunity, the virus can spread quickly throughout a population, sometimes even leading to an outbreak. Herd immunity, however, gives the disease very little chance of spreading because those who have immunity are less likely to transmit it to others. This process protects the most vulnerable community members, including babies who are too young to receive vaccinations and those with compromised immune systems who may have difficulty fighting the disease. In this case, the pathogen will be less likely to reach the vulnerable individuals because they compose a smaller portion of the population.
