The immune system is responsible for defending the body.
It includes all the elements of the lymphatic system (vessels, lymphatic nodes and organs), as well as all the other elements that help defend the body, such as white blood cells.
The human immune system has two defence mechanisms:
Specific defence mechanisms, or specific immunity, are mechanisms the human body uses to protect itself against a particular antigen.
Some white blood cells have the ability to produce antibodies specifically designed to neutralize a particular type of antigen. For example, antibodies to fight the flu virus aren’t the same as those that neutralize the bacteria causing bronchitis. This type of immunity isn’t innate and must be acquired at some point.
There are two types of specific immunity.
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Active immunity is acquired by making antibodies to defend the body against antigens that appear during an infection or as a result of vaccination.
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Passive immunity is transmitted from the mother to the fetus through antibodies from the placenta. It is also transmitted through breast milk given in the first days after childbirth.
There are two main ways to acquire active immunity to an antigen.
Antibody production following an infection
Immunity can be acquired naturally by producing antibodies against specific antigens after an infection in the body, such as after contracting chickenpox or the mumps.
Antigens are substances that are foreign to the human body and that trigger an immune response.
When these antigens enter the body, white blood cells make antibodies against them.
Antibodies are substances secreted by white blood cells that can neutralize a specific antigen.
After producing these antibodies, the white blood cells remember how to produce them again, when needed to fight this particular type of antigen. If a second infection from the same antigen occurs, the white blood cells quickly respond by producing the appropriate antibodies, preventing a second occurrence of the disease.
Antibody production following vaccination
Immunity can also be acquired artificially by injecting a vaccine.
A vaccine is a substance that stimulates the immune system to make antibodies that immunize the organism against a particular disease.
The vaccine contains weakened or inactive antigens that cause the white blood cells to produce antibodies. As a result, the white blood cells remember the type of antibody related to the type of antigen injected. If this type of antigen enters the body, the white blood cells can react and produce the appropriate antibodies.
Non-specific defence mechanisms, or non-specific immunity, are the innate mechanisms that always act in the same way, no matter what pathogen they come in contact with.
Non-specific defence mechanisms include structures that rapidly defend the body against infectious agents. These structures include the skin, mucous membranes (such as those lining the inside of the nasal cavity) and various bacterial flora found throughout the body (in the mouth, stomach and vagina, for example). They act like barriers to prevent pathogens from moving further into the body.
However, if a pathogen manages to cross these barriers, there are internal non-specific defence mechanisms, such as diapedesis and phagocytosis.
Diapedesis is the response of a white blood cell when an infection is detected, for example. The cell leaves the blood vessel in which it’s circulating to slip between the surrounding cells and reach the infection site.
At the infection site, the white blood cell engages in phagocytosis. The activity of white blood cells can cause the tissue to become inflamed, causing redness, swelling, heat and pain.
Phagocytosis is the process of a white blood cell surrounding the antigen to destroy it.
In other words, white blood cells can capture and ingest antigens to destroy them. When a white blood cell encounters a pathogen, it surrounds it with cytoplasm extensions, called pseudopods. At the end of phagocytosis, a series of reactions occur inside the white blood cell to form digestive waste that will eventually be expelled from the body.
