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Immune System: Overview in Humans

There are two types of immune response: innate immunity and adaptive immunity. Both types work together to keep you healthy. The cells of the human immune system originate from stem cells in important key organs in the body: early in life from the liver, and later the bone marrow. The bone marrow continues to produce new cells throughout a person’s lifetime. These organs provide a place for specialized cells of the immune system to mature and develop. From here they go out into the blood stream and to other organs of the body to where they will spring into action when needed.

Innate immunity
Innate immunity is a very quick response to danger signals that are common among microbes (usually bacteria, viruses, and parasites). This is the first line of defense when a microbe enters the body. When you cut yourself, or have food poisoning, your innate immunity responds right away. Some cells, like phagocytic ones, “eat” bacteria and other organisms. Phagocytic cells migrate throughout the body or stay in organs and tissues to wait for invading bacteria or other invading microbes. When they find one, they trap it on their surface, then ingest it (bring it into the cell) and destroy it. Some microbes have developed special outer surfaces that make it hard for the phagocytic cell to trap them. Humans share innate immune components with Drosophila.

Phagocytic immune cell "eating" bacterial invaders
Phagocytic cells neutralize or destroy invading bacteria and other microbes. They do this by trapping the microbe on their surface, then ingesting it (bringing it into the cell and eating it), and destroying it with enzymes. Some microbes have developed special outer surfaces that make it hard for the phagocytic cell to trap them. The phagocytic cells in Drosophila are called haemocytes; in humans they are called macrophages and neutrophils.

Adaptive Immunity
In humans, and vertebrates in general, immune cells are also able to learn and improve immune defenses when they encounter the same microbe several times. This part of the immune system is called the adaptive immune response and this ability is called a “memory response”. Specialized cells work together to recognize a disease-causing microbe and they create targeted responses to the organism. The key to this response is the production of specific antibodies by the cells that are designed to specifically attack the microbe causing the threat. These responses will be “remembered” by the cells so that they can respond quickly the next time this threat is encountered.

Diagram showing progression from macrophage that has encountered virus to antigen presenting cell to T cell to B cell to plasma cell to antibodies neutralizing virus. T cells and B cells also give rise to memory cells.
The key to the adaptive humoral response is the production of specific antibodies by cells that are designed to attack a certain microbe. The production of specific antibodies involves complex cooperation between immune cells. First, the macrophage traps virus particles. It processes these and becomes an antigen presenting cell (APC). The APC then presents the processed virus to a Helper T cell, which becomes activated and undergoes clonal expansion. This means that it makes more of itself (Memory Helper T cells that will remember the virus target) and causes the B cell to become a plasma cell that makes the antibody against the virus. The antibodies attach to the virus to destroy it. The B cell will also clonally expand to make Memory B cells that will remember which antibody to make against this virus. Memory cells wait for future invasions by this virus and respond quickly.

Application of Adaptive Immunity: Vaccines
The memory response is how vaccines work (target viruses) and also why we usually do not get the same cold or flu twice. The cartoon showing how specific antibodies are made demonstrates how vaccines work too. Vaccines are made from killed virus or parts of viruses that are injected into the body so that your immune system cells can start making antibodies against them. Several immune cells work together to create the memory response that results in specific antibodies. That way, if you are infected with the real virus your immune system will be ready to respond. Vaccines have been critical in reducing the amount of diseases, like polio and small pox, in the world’s population. Cold and flu viruses are very good at mutating, however, and they can fool the immune cells to cause a re-infection.

What if the system doesn’t work very well or is missing specialized cells? Illness results. Exposure to stressors can cause your immune system to malfunction. Not getting enough sleep or exercise can leave your immune system more vulnerable to attack. Astronauts are exposed to the stress of microgravity during spaceflight, and it is important that we fully understand how their immune response changes as a result of space travel. What we learn may be beneficial for people on Earth, including those who are born with Severe Combined Immunodeficiency, have AIDS, and cancer therapy patients.

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 FirstGov  NASA
Editor: Carol Elland
NASA Official: B J Navarro
Last Updated: September 2006
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