Taking a look at the function of the immune system and the body’s response to vaccinations in eliciting an immune response.
Written by: Shriya
In the previous articles of this blog series, we gained a general knowledge of what vaccines are. Now that we know more about the different types of vaccines, we can learn about the technicalities of how they work. To understand how vaccines work, it helps to first look at the immune system, which is how the body fights illness.
When germs, such as bacteria or viruses, invade the body, they attack and multiply. As they invade the body, there is an infection and a resulting illness. Your immune system is quick to recognize the pathogen as invaders. This is because the proteins or sugars on the bacteria’s surface have different shapes to any of the cells in the human body. They trigger a complex chain of events involving many different types of white blood cells working together.
One type of white blood cell is able to make antibodies to fight the invaders. Antibodies can stick to the proteins or sugars on the bacteria’s surface, and this kills the bacteria or disables them. However, not all antibodies will work against these bacteria, since they have to be exactly the right shape. The body has a library of billions of white blood cells, each of which can make just one shape of antibody. Only a few of these antibodies will match the invading bacteria. (1) Producing antibodies of the right shape can take several days. By this time there could be billions of disease-causing bacteria in your body. Once the right cells are activated they quickly divide and make masses of antibodies that stick to the bacteria. Eventually, your body gets rid of all the bacteria and you recover.
Antibodies remain in the blood, and some white blood cells may also become ‘memory cells’. If those specific bacteria invade the body again, the immune system will respond so quickly that you won’t get ill. (1)
Vaccines work in the same way, mimicking disease agents and stimulating the immune system to build defenses. They contain weakened or dead bacteria or viruses, or even just a few proteins or sugars from the surface. Vaccination exposes the body to antigens that are similar to the antigens found on a pathogen. By posing as a specific pathogen, the vaccine primes the immune system to respond with speed and strength if the body encounters the pathogen in the future.
Now that we understand how the immune system functions, we can take a look at the step by step process of the body’s response to vaccines.
First, we should familiarize ourselves with a few key definitions:
Antigen: anything that is foreign to the human body, including viruses and bacteria
Antigen-Presenting Cell (APC): a large group of various cells that trigger the cellular immune response by processing an antigen and exposing it in a form recognizable by T
cells in the process known as antigen presentation. (2)
Antibody: a protein component of the immune system that circulates in the blood,
recognizes foreign substances like bacteria and viruses, and neutralizes them. After exposure to an antigen, antibodies continue to circulate in the blood, providing protection against future exposures to that antigen. (3)
Naive T Cells: white blood cells originating in the thymus that function to respond to
antigens the immune system has never encountered, recognize cognate antigens and
initiate an immune response, and enable the immune system to respond to new pathogens
(4)
Killer T-cells: find and destroy infected cells by injecting them with special enzymes that
destroy its nucleus and its structure.
T Helper Cells: use chemical messages to give instructions to the other immune system cells. These instructions help Killer T-cells and B-cells multiply so they can fight the infection and make sure the fight stays under control. (5)
Naive B Cells: white blood cells, originating in the bone marrow, that secrete antibodies in response to antigens that the immune system has never encountered
Plasma B Cells: secrete specific antibodies and initiate reactions that destroy antigens, though only living for a few days.
Memory Cells: recognize and respond to previously encountered antigens to provide a
faster and stronger immune response after encountering the same antigen.
Response to Vaccines: (6)
Capture by Antigen-Presenting Cell: Antigen-Presenting Cells roam the body to search for invaders. When an APC finds the vaccine antigen, it ingests the invader, breaks it apart, and displays a piece of the antigen on its surface.
T Helper Cell Activation: APCs displaying the antigen travel to areas where immune cells cluster, such as lymph nodes. Naive T cells specific to the antigen recognize it as foreign and become activated. T helper cells alert nearby cells about the presence of the invader.
B Cell Activation: Naive B cells react to the vaccine antigen when it enters the body. Active B cells undergo cell division, providing more active B cells that are specific to the vaccine antigen. Some of these mature into plasma B cells and others develop into memory B cells.
B Cells Mature into Plasma B cells: After activation by the vaccine antigen and signals from activated T helper cells, some B cells transform into plasma B cells that produce antibodies specific to the vaccine antigen.
Plasma B Cells Secrete Antibodies: “Y” shaped proteins called antibodies are released at high levels every second.
Antibodies Bind to Specific Antigens: Each antibody attaches to one specific, target antigen. This action may prevent the antigen from entering a cell or mark the antigen for destruction.
Killer T Cell Response: Killer T cells find and destroy the cells invaded by the vaccine virus. Naive killer T cells require an APC to display an antigen piece before they are activated.
Retention of Memory Cells: The goal of immunization is to produce memory of the vaccine antigen through a large population of memory cells.
Vaccinations “program” the immune system to remember a particular disease agent by allowing it to “practice” on a weakened or killed version of the pathogen. This is called a primary response to a pathogen.
If the pathogen invaded the body again in full strength, the immune system is ready to respond with a swift and specific defense. This is called a secondary response to a pathogen. Secondary responses happen faster and at a greater magnitude than primary responses, resulting in the creation of more antibodies to fight the pathogen and more memory cells to fight in the future.
Response to Pathogen after Vaccination: (6)
Infection: A pathogen enters the body. Antigen-presenting cells ingest it, displaying positions of the antigen on their surface.
Activation of Memory Cells: Memory T cells created during the vaccination process encounter the APCs and recognize the antigen they are displaying. Then, they release signals to alert other immune cells and encourage a response. The presence of the pathogen also reactivates memory B cells.
Memory B Cells Become Active Plasma Cells: Memory B cells respond to the presence of an antigen by activating and differentiating into plasma B cells. The plasma B cells produce and secrete antibodies specific to the antigen that activated them. In the secondary response, the plasma cells produce more antibodies at a faster rate than in the primary response.
Antibodies Attack the Pathogen: The antibodies bind to the surface of the pathogen. This may prevent the pathogen from entering a cell or mark it for removal or destruction by other cells of the immune system.
Killer T Cells Response: If the vaccination process induced a killer T cell response, then memory cells of that type will persist and be activated by exposure to the antigen.
Retention of Memory Cells: The invading pathogen has been stopped. As with the original vaccination, some memory B and T cells remain to guard against any future attacks by the same pathogen.
Vaccines help develop immunity by imitating an infection. This type of infection, however, almost never causes illness, but it does cause the immune system to produce immune cells and antibodies. Thus, the body builds immunity that will prevent infection in the future.
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