Immune System

The immune system is the body’s defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade our systems and cause disease.

Immune Attack’s content includes three types of white blood cells:


monocytes
macrophages
neutrophils

What are monocytes? Monocytes are large white blood cells. Monocytes migrate into the connective tissue, where they become macrophages and move to kill invading enemies.
How do monocytes exit the blood vessels? Monocytes exit the blood vessel by "squeezing" between endothelial cells (the cells that line the blood vessel).
Where do monocytes exit the blood vessels? Once out of the blood vessel network, monocytes (now called macrophages) move toward the infecting cells (e.g., bacteria) and attempt to find and consume the invaders, one cell at a time.
What happens once a monocyte exits the blood vessel network? Once out of the blood vessel network, monocytes (now called macrophages) move toward the infecting cells (e.g., bacteria) and attempt to find and consume the invaders, one cell at a time.

What is a macrophage? A macrophage is a type of white blood cell that eats bacteria as part of the immune response to infection. The name "macrophage" means "big eater." Macrophages are versatile cells; besides acting as scavengers, they secrete a wide variety of chemical signals that alert other white blood cells of invaders.
What do macrophages do? Macrophages live in tissues, and when they encounter invading bacteria or other attackers they engulf and destroy them. Macrophages also release chemicals to alert neutrophils and direct them to the site of attack to help fight the infection. Macrophages start as monocytes in the blood vessel network. They become macrophages once they leave the bloodstream and enter the connective tissue.
What is the difference between monocytes and macrophages? Macrophages begin as monocytes, white blood cells that start their life cycle in the bone marrow. When a monocyte exits the blood vessel and enters the tissue, it undergoes a series of changes and becomes a macrophage.
How do monocytes interact with selectin and ICAM? In order for monocytes to reach the site of an infection, they must leave the blood vessel and enter the connective tissue. To do so, monocytes must adhere to the cells that line the blood vessel wall and then exit the blood vessel. Selectins help slow the monocyte, and ICAMs help the monocyte adhere and exit the blood vessel.
How do macrophages become active? Inactive macrophages flowing in the blood are called monocytes. When they get a signal that an infection is near (by grabbing an ICAM molecule for example), they are activated to leave the blood vessel to search and destroy invaders. Macrophages patrolling outside the blood vessels are activated by signals on the outside of bacteria and other invaders.
How does a macrophage get out of the blood vessel? The chemicals released by an infection make blood vessels swell up and become leaky. This is inflammation, and it usually hurts. The inflammation also causes the blood vessels to produce a molecule called selectin and a molecule called ICAM that stick out on the inside of the swollen vessels. Monocytes flowing with the blood grab selectin and slow down. When the monocytes are moving slowly enough they can engage the ICAM which acts like an on switch causing the passive monocytes to become active killer macrophages. These activated macrophages are able to crawl out through gaps between the cells lining the blood vessel wall and start heading for the infection.
How do macrophages tell friend from foe? Many invading enemies like bacteria and viruses have markers on their surfaces. These markers are different from anything on the human body's cells. Macrophages recognize these markers as signs of foreign invaders and kill the cells carrying them.
How does a macrophage track invaders? Macrophages follow trails of chemicals (floating in the body's fluids) that lead to the site of an alien attack. You do this when you smell something burning and use your nose to follow the chemical trail to where the smell is strongest.
How do a macrophages kill? Macrophages wrap around bacteria or other types of invaders, engulfing them in a special chamber. The chamber then fills with acid, dissolving the pathogen.
How many macrophages are in the body? There are about 2 billion (2 x 10^9) monocytes in an adult body. The number of macrophages depend on the amount of infection within the body.
How long do macrophages live? Macrophages can live for several months in tissues and can be activated at any time.
What receptors are on a macrophage's surface? Macrophages have many types of receptors on their surface, but the Toll-like receptors (TLRs) are most important. TLRs are the receptors that allow macrophages to recognize (and thus, target) invaders like bacteria and viruses.
Where does the name macrophage come from? The name macrophage means "big eater" in Greek ("macro" = big; "phage" = eat).

What are neutrophils? Neutrophils are a type of white blood cell. As part of the innate immune system, they destroy many kinds of invading bacteria and other types of attackers that carry markers showing whether they are friends or foes. Neutrophils attack very quickly and in large numbers.
How are neutrophils activated? Neutrophils are activated by chemical signals sent by invader-fighting macrophages. Signals can also be sent by invaders themselves.
How do neutrophils get out of the blood vessel? The chemicals released by macrophages fighting an invader make blood vessels swell up and become leaky. This swelling is inflammation, and it usually hurts. A molecule called selectin sticks out on the inside of the swollen vessels. Neutrophils flowing with the blood grab selectin and slow down. The swelling forces gaps between the cells that leak fluid from the blood into tissue, then the neutrophils crawl out through these gaps.
How do neutrophils tell friend from foe? Many invading enemies, such as bacteria and viruses, have markers on their surfaces that are different from anything on the human body's cells. Neutrophils recognize these markers as signs of foreign invaders and kill the cells carrying them.
How do neutrophils track invaders? Neutrophils follow trails of chemicals floating in the body's fluids that lead to the site of an alien attack.
How do neutrophils kill? Neutrophils surround pathogens (bacteria or other invaders) by wrapping around and engulfing them. The engulfed pathogen ends up within a special chamber inside of the neutrophil; the chamber then fills with acid and the pathogen is dissolved.
How many neutrophils are in the body? There are about 25 billion (25 x 10^9) neutrophils in an adult human body.
Where are neutrophils created? Neutrophils are made in the bone marrow that's inside most bones. Special cells in the marrow called stem cells are produce all types of blood cells ? including Neutrophils. Newly made Neutrophils leave through blood vessels going through the bone.
How long do neutrophils live? Neutrophils only live for a few days. Once neutrophils are active at the site of an infection they die of exertion after a few kills.
What happens when neutrophils die? Dead neutrophils become the pus that oozes from many infections. They are eventually eaten by macrophages.
What receptors are on a neutrophil's surface? Neutrophils have surface receptors that allow them to recognize markers that are common to many bacteria and other invaders. Neutrophils also have surface receptors that recognize chemicals released by macrophages.
Where does the name neutrophil from? When different blood cells were first discovered, they were named for the way they changed colors when certain chemicals were poured on them. Neutrophils' color changed in a way that was considered “neutral."


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