In this lesson, students will learn
The immune system is the body’s defense against infections. The immune system attacks germs and helps keep us healthy.
Pathogens can evolve rapidly and adapt. This helps them avoid detection and neutralization by the immune system. However, different defense mechanisms have evolved in order to recognize and neutralize pathogens. The immune system is even possessed by simple unicellular organisms like bacteria in the form of enzymes to protect them against bacteriophage infections. Some basic immune mechanisms evolved in ancient eukaryotes and still remain in their modern descendants like invertebrates and plants. Some of these mechanisms are phagocytosis, the complement system and antimicrobial peptides known as defensins. Jawed vertebrates like humans have more sophisticated defense mechanisms which include the ability to adapt with time and recognize specific pathogens more efficiently.
Largely, humans have two types of immunity — innate, and adaptive. There is another type of temporary immunity known as "passive" immunity which we will explain later.
Innate immunity
Innate immunity is the immune system you're born with and mainly consists of barriers on and in the body that keep foreign threats out. Components of innate immunity include skin, stomach acid, enzymes found in tears and skin oils, mucus and the cough reflex. There are also chemical components of innate immunity, including substances called interferon and interleukin-1. Innate immunity is non-specific, meaning it doesn't protect against any specific threats.
Innate immunity consists of:
Adaptive immunity
Adaptive, or acquired, immunity targets specific threats to the body. Adaptive immunity is more complex than innate immunity. In adaptive immunity, the threat must be processed and recognized by the body, and then the immune system creates antibodies specifically designed to the threat. After the threat is neutralized, the adaptive immune system "remembers" it, which makes future responses to the same germ more efficient. We develop adaptive immunity when we're exposed to diseases or when we're immunized against them with vaccines.
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Passive immunity
Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk give a baby temporary immunity to diseases the mother has been exposed to.
White blood cells
Many cells and organs work together to protect the body. White blood cells, also called leukocytes play an important role in the immune system. Some types of white blood cells called phagocytes chew up invading organisms. Others called lymphocytes, help the body remember the invaders and destroy them.
One type of phagocyte is the neutrophil which fights bacteria. When someone might have a bacterial infection, doctors can order a blood test to see if it caused the body to have lots of neutrophils. Other types of phagocytes do their own jobs to make sure that the body responds to invaders.
The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and either stay there and mature into B cells or go to the thymus gland to mature into T cells. B lymphocytes are like the body's military intelligence system — they find their targets and send defenses to lock onto them. T cells are like the soldiers — they destroy the invaders that the intelligence system finds.
Antibodies
Antibodies help the body to fight microbes or the toxins (poisons) they produce. They do this by recognizing substances called antigens on the surface of the microbe, or in the chemicals they produce, which mark the microbe or toxin as being foreign. The antibodies then mark these antigens for destruction. There are many cells, proteins, and chemicals involved in this attack.
Lymphatic system
It is a network of delicate tubes throughout the body. The main roles of the lymphatic system are to:
The lymphatic system is made up of:
Spleen
The spleen is a blood-filtering organ that removes microbes and destroys old or damaged red blood cells. It also makes disease-fighting components of the immune system (including antibodies and lymphocytes).
Bone marrow
Bone marrow is the spongy tissue found inside your bones. It produces the red blood cells our bodies need to carry oxygen, the white blood cells we use to fight infection, and the platelets we need to help our blood clot.
Thymus
The thymus filters and monitors your blood content. It produces white blood cells called T-lymphocytes.
Types of Immunity Cells
The immune system has cells that perform specific functions. These cells are found in the bloodstream and are called white blood cells.
B cells - B cells are also called B lymphocytes. These cells produce antibodies that bind to antigens and neutralize them. Each B cell makes one specific type of antibody. For example, there is a specific B cell that helps to fight off the flu.
T cells - T cells are also called T lymphocytes. These cells help to get rid of good cells that have already been infected.
Helper T cells - Helper T cells tell B cells to start making antibodies or instruct killer T cells to attack.
Killer T cells - Killer T cells destroy cells that have been infected by the invader.
Memory cells - Memory cells remember antigens that have already attacked the body. They help the body to fight off any new attacks by a specific antigen.
A rise in body temperature or fever can happen with some infections. This is actually an immune system response. A rise in temperature can kill some microbes. Fever also triggers the body's repair process.
When the body senses foreign substances called antigens, the immune system works to recognize the antigens and get rid of them.
Layered defense is a type of defense where physical barriers are used to prevent an organism from pathogens like viruses and bacteria from entering an organism. In case a pathogen breaches these barriers, the innate immune system gives an immediate and non-specific response. Innate immune systems are found in all animals and plants. In case pathogens evade the innate response, vertebrates have a second protection layer known as the adaptive immune system. This is activated by the innate response.
B-lymphocytes are triggered to make antibodies. These specialized proteins lock onto specific antigens. The antibodies stay in a person’s body. That way, if the immune system encounters that antigen again, the antibodies are ready to do their job. That’s why someone who gets sick with a disease, like chickenpox, usually won’t get sick from it again.
This is also how immunizations (vaccines) prevent some diseases. An immunization introduces the body to an antigen in a way that doesn’t make someone sick. But it does let the body make antibodies that will protect the person from future attacks by the germ.
Although antibodies can recognize an antigen and lock onto it, they can’t destroy it without help. That’s the job of the T-cells. They destroy antigens tagged by antibodies or cells that are infected or somehow changed. T-cells can also help signal other cells (like phagocytes) to do their jobs.
Antibodies also can
These specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.
Vaccines introduce microbes that are already killed or modified so we don't get sick. However, the immune system doesn't know this. It builds up defenses and antibodies against the disease. When the real disease tries to attack, our body is ready and can quickly neutralize the antigens.
It is common for people to have an over or underactive immune system.
Over-activity of the immune system can take many forms, including
Underactivity of the immune system, also called immunodeficiency can
An underactive immune system does not function correctly and makes people vulnerable to infections. It can be life-threatening in severe cases.
People who have had an organ transplant need immunosuppression treatment to prevent the body from attacking the transplanted organ.
