To understand how vaccines work, it helps to first look at how the body fights illness. When germs, such as bacteria or viruses, invade the body, they attack and multiply. This invasion, called an infection, is what causes illness. The immune system uses several tools to fight infection. Blood contains red blood cells, for carrying oxygen to tissues and organs, and white or immune cells, for fighting infection.
The Immune System: The Body’s Defense Against Infection
As long as your immune system is running smoothly, you don’t notice that it’s there. But if it stops working properly – because it’s weak or can't fight particularly aggressive germs – you get ill. Germs that your body has never encountered before are also likely to make you ill. Some germs will only make you ill the first time you come into contact with them. These include childhood diseases like chickenpox.
The tasks of the immune system
Without an immune system, we would have no way to fight harmful things that enter our bodies from the outside or harmful changes that occur inside our bodies. The main tasks of the body’s immune system are to fight disease-causing germs (pathogens) like bacteria, viruses, parasites or fungi, and to remove them from the body, to recognize and neutralize harmful substances from the environment, and
to fight disease-causing changes in the body, such as cancer cells.
How is the immune system activated?
The immune system can be activated by a lot of different things that the body doesn’t recognize as its own. These are called antigens. Examples of antigens include the proteins on the surfaces of bacteria, fungi, and viruses. When these antigens attach to special receptors on the immune cells (immune system cells), a whole series of processes are triggered in the body. Once the body has come into contact with a disease-causing germ for the first time, it usually stores information about the germ and how to fight it. Then, if it comes into contact with the germ again, it recognizes the germ straight away and can start fighting it faster.
The body’s own cells have proteins on their surface, too. But those proteins don’t usually trigger the immune system to fight the cells. Sometimes the immune system mistakenly thinks that the body's own cells are foreign cells. It then attacks healthy, harmless cells in the body. This is known as an autoimmune response.
The innate and adaptive immune system
There are two subsystems within the immune system, known as the innate (non-specific) immune system and the adaptive (specific) immune system. Both of these subsystems are closely linked and work together whenever a germ or harmful substance triggers an immune response.
The innate immune system provides a general defense against harmful germs and substances, so it’s also called the non-specific immune system. It mostly fights using immune cells such as natural killer cells and phagocytes (“eating cells”). The main job of the innate immune system is to fight harmful substances and germs that enter the body, for instance through the skin or digestive system.
The adaptive (specific) immune system makes antibodies and uses them to specifically fight certain germs that the body has previously come into contact with. This is also known as an “acquired” (learned) or specific immune response because the adaptive immune system is constantly learning and adapting, the body can also fight bacteria or viruses that change over time.
The immune system and microbial infection
The immune system keeps a record of every microbe it has ever defeated, in types of white blood cells (B- and T-lymphocytes) known as memory cells. This means it can recognize and destroy the microbe quickly if it enters the body again before it can multiply and make you feel sick.
Some infections, like the flu and the common cold, have to be fought many times because so many different viruses or strains of the same type of virus can cause these illnesses. Catching a cold or flu from one virus does not give you immunity against the others. The main parts of the immune system are:
- white blood cells
- complement system
- lymphatic system
- bone marrow
White blood cells
White blood cells move through blood and tissue throughout your body, looking for foreign invaders (microbes) such as bacteria, viruses, parasites, and fungi. When they find them, they launch an immune attack. White blood cells include lymphocytes (such as B-cells, T-cells and natural killer cells), and many other types of immune cells.
The body's other defenses against microbes
As well as the immune system, the body has several other ways to defend itself against microbes, including:
- skin - a waterproof barrier that secretes oil with bacteria-killing properties
- lungs - mucous in the lungs (phlegm) traps foreign particles, and small hairs (cilia) wave the mucous upwards so it can be coughed out
- digestive tract - the mucous lining contains antibodies, and the acid in the stomach can kill most microbes
- other defenses - body fluids like skin oil, saliva and tears contain anti-bacterial enzymes that help reduce the risk of infection. The constant flushing of the urinary tract and the bowel also helps.
How Vaccines Work
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 T-lymphocytes and antibodies. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever. Such minor symptoms are normal and should be expected as the body builds immunity.