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We are surrounded by millions of bacteria, viruses and other microbes that have the potential to enter our bodies and cause harm. The immune system is the body's defence against pathogens (disease-causing microbes). The immune system is made up of non-specialised defences such as skin and the acidic juice produced by your stomach. But it also has some highly specialised defences which give you immunity against (resistance to) particular pathogens. These defences are special white blood cells called lymphocytes. Other types of white blood cells play an important part in defending your body against infection. The lymphatic system is also part of the immune system. The lymphatic system is made up of a network of vessels (tubes) which carry fluid called lymph. It contains specialised lymph tissue and all of the structures dedicated to the production of lymphocytes. The immune system is generally divided into two parts. The first part is the defences you are born with. These form what are known as the innate system. The second part of your immune system, known as immunity, develops as you grow. Your immunity gives you protection against specific pathogens. Not only can this system recognise particular pathogens, it also has a memory of this. This means that if you encounter a certain pathogen twice, your immune system recognises it the second time around. This usually means your body responds quicker to fight off the infection. The innate system is found in many different places around the body. First line of defence is your skin. Skin forms a waterproof barrier that prevents pathogens from entering the body. Your body cavities, such as the nose and mouth, are lined with mucous membranes. Mucous membranes produce sticky mucus which can trap bacteria and other pathogens. Other fluids produced by the body help to protect your internal layers from invasion by pathogens. Gastric juice produced by the stomach has high acidity which helps to kill off many of the bacteria in food. Saliva washes pathogens off your teeth and helps to reduce the amount of bacteria and other pathogens in your mouth. If bacteria or other pathogens manage to get through these initial defences, they encounter a second line of defence. Most of these defences are present in your blood, either as specialised white blood cells or as chemicals released by your cells and tissues. The second part of your immune system, the part that gives you immunity, involves the activation of lymphocytes. This will be described later on. Lymphocytes are found in your blood and also in specialised lymph tissue such as lymph nodes, the spleen and the thymus. The first line of defence is your body's skin and mucous membranes, as mentioned above. If pathogens manage to get through these barriers, they encounter special white blood cells present in your bloodstream. There are different types of white cells, called neutrophils, lymphocytes, eosinophils, monocytes, and basophils. White blood cells travel in the bloodstream and react to different types of infection caused by bacteria, viruses or other pathogens. Neutrophils engulf bacteria and destroy them with special chemicals. Eosinophils and monocytes also work by swallowing up foreign particles in the body. Basophils help to intensify inflammation (swelling). Inflammation is part of your body's immune response. Damage to your tissues causes the release of chemicals into the blood. These chemicals make blood vessels leaky, helping specialised white blood cells get to where they are needed. They also attract neutrophils and monocytes to the site of the injury, which helps to protect against a bacterial infection developing.


One of the key processes of inflammation is the transmigration of circulating leukocytes across the endothelium. Among the leukocytes, neutrophils and monocytes are large phagocytes that can respond quickly to infection or injury. On sensing danger, neutrophils and monocytes adhere to the endothelium and transmigrate to the adjacent tissue via the coordinated activities of adhesion molecules, integrins, cytokines, and chemokines. Once they accumulate, these myeloid cells participate in a myriad of immune inflammatory activities. The importance of this event cannot be understated, especially because the accumulation of leukocytes in tissue is a double-edged sword. On the one hand, coordinated leukocyte accumulation in injured or infected sites is required for effective pathogen elimination and tissue healing. On the other hand, uncontrolled accumulation is a defining feature of chronic diseases, such as atherosclerosis. Understanding leukocyte migration is essential to understanding the immune system. Neutrophils and monocytes do not accumulate all at once. In a typical acute inflammatory response, there is a well-defined sequence: neutrophils accumulate first; monocytes accumulate second. Among the monocytes, of which less than 2 subsets circulate in the mouse and human, there is yet another sequence: inflammatory murine monocytes accumulate first and reparative monocytes accumulate second. This temporal hierarchy of accumulation is likely required for an effective innate response. The subsets, which have overlapping but also specialized functions, contribute sequentially to processes that involve pathogen elimination, efferocytosis, restoration of tissue integrity, and the initiation of adaptive immunity. How are these accumulation sequences orchestrated? Although the molecules that guide leukocyte arrest, firm adhesion, and transmigration can explain the process in mechanistic detail, they do not reveal the larger context of influence among leukocyte classes.


Defensive properties of neutrophils and monocyte and macrophages. Once produced, neutrophils spend 4-8 hours in blood, then enter tissue & survive for 4-5 days. Once produced, monocytes spend 10-20 hours in blood, then enter tissue, swell up and mature into macrophages and survive for months. Both neutrophils and monocytes phagocytoze foreign and unwanted material. They can exit blood and enter tissue via diapedesis. They move to the site of infection by chemotaxis, which is caused by chemical substances, e.g. Bacterial or viral toxins, tissue debris from inflamed tissue etc. Macrophage and Neutrophil response during inflammation: 1st line of defense: within minutes after inflammation begins, tissue macrophages begin their phagocytic actions. Many of them break loose from their site of attachments and become mobile in response to chemotactic factors. 2nd line of defense: large numbers of neutrophils move to inflamed area as a result of chemotaxis caused by products in inflamed tissues. Number of neutrophils increases rapidly after a few hours (neutrophilia); this is caused by inflammatory products that are transported through blood to bone marrow, causing neutrophils to be mobilized and moved into circulating blood. 3rd line of defense: a second macrophage invasion of inflamed tissue. Along with neutrophils, monocytes enter inflamed tissue and enlarge to become macrophages. Build-up of macrophages in inflamed tissue is much slower than that of neutrophils because number of monocytes in blood is low and storage pool in bone marrow is much less than that of neutrophils. 4th line of defense: stimulation of granulocytic and monocytic progenitor cells of bone marrow. This takes 3-4 days for newly formed granulocytes & monocytes to leave the bone marrow.


We are surrounded by millions of bacteria, viruses and other microbes that have the potential to enter our bodies and cause harm.

The immune system is the body's defence against pathogens (disease-causing microbes).

The immune system is made up of non-specialised defences such as skin and the acidic juice produced by your stomach.

But it also has some highly specialised defences which give you immunity against (resistance to) particular pathogens.

These defences are special white blood cells called lymphocytes.

Other types of white blood cells play an important part in defending your body against infection.

The lymphatic system is also part of the immune system.

The lymphatic system is made up of a network of vessels (tubes) which carry fluid called lymph.

They can exit blood and enter tissue via diapedesis.

The immune system is generally divided into two parts.

The first part is the defences you are born with.

These form what are known as the innate system.

The second part of your immune system, known as immunity, develops as you grow.

Your immunity gives you protection against specific pathogens.

Not only can this system recognise particular pathogens, it also has a memory of this.

This means that if you encounter a certain pathogen twice, your immune system recognises it the second time around.

This usually means your body responds quicker to fight off the infection.

The innate system is found in many different places around the body.

First line of defence is your skin.

Skin forms a waterproof barrier that prevents pathogens from entering the body.

Your body cavities, such as the nose and mouth, are lined with mucous membranes.

Mucous membranes produce sticky mucus which can trap bacteria and other pathogens.

Other fluids produced by the body help to protect your internal layers from invasion by pathogens.

Gastric juice produced by the stomach has high acidity which helps to kill off many of the bacteria in food.

Saliva washes pathogens off your teeth and helps to reduce the amount of bacteria and other pathogens in your mouth.

If bacteria or other pathogens manage to get through these initial defences, they encounter a second line of defence.

Most of these defences are present in your blood, either as specialised white blood cells or as chemicals released by your cells and tissues.

The second part of your immune system, the part that gives you immunity, involves the activation of lymphocytes.

This will be described later on.

Build-up of macrophages in inflamed tissue is much slower than that of neutrophils because number of monocytes in blood is low and storage pool in bone marrow is much less than that of neutrophils.

The first line of defence is your body's skin and mucous membranes, as mentioned above.

If pathogens manage to get through these barriers, they encounter special white blood cells present in your bloodstream.

There are different types of white cells, called neutrophils, lymphocytes, eosinophils, monocytes, and basophils.

White blood cells travel in the bloodstream and react to different types of infection caused by bacteria, viruses or other pathogens.

Neutrophils engulf bacteria and destroy them with special chemicals.

Among the monocytes, of which less than 2 subsets circulate in the mouse and human, there is yet another sequence: inflammatory murine monocytes accumulate first and reparative monocytes accumulate second.

This takes 3-4 days for newly formed granulocytes & monocytes to leave the bone marrow.

Both neutrophils and monocytes phagocytoze foreign and unwanted material.

Defensive properties of neutrophils and monocyte and macrophages.

Eosinophils and monocytes also work by swallowing up foreign particles in the body.

Basophils help to intensify inflammation (swelling).

Inflammation is part of your body's immune response.

Damage to your tissues causes the release of chemicals into the blood.

These chemicals make blood vessels leaky, helping specialised white blood cells get to where they are needed.

Although the molecules that guide leukocyte arrest, firm adhesion, and transmigration can explain the process in mechanistic detail, they do not reveal the larger context of influence among leukocyte classes.