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Pathogens and the immune system

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The non-specific immune system

Every day you meet millions of pathogens in your daily life – why aren’t you constantly ill? The human body has many different defence mechanisms against pathogens. They all depend on recognising foreign material and destroying or, at the very least, inactivating the pathogen.

  • The non-specific responses of the body react to any invading pathogen.
  • The specific immune responses each respond to specific pathogens.

Defending the body

The human body has a number of adaptations that either prevent the easy entry of pathogens into the body or destroy pathogens as soon as they get inside, before they can cause an infection. For example:

  1. The skin forms a relatively impenetrable barrier and is covered in a healthy flora of microorganisms that out-compete most pathogens
  2. Tears contain the enzyme lysozyme which causes bacteria that land on the eyes to burst (lyse).
  3. The stomach contains acid that destroys most pathogens that enter through the mouth
  4. The gas exchange system produces mucus that traps pathogens, and is then moved out into the throat or nose by cilia beating on the lining of the tubes.
  5. If the skin is broken, the blood clots and then a scab forms. This prevents you bleeding to death and prevents the entry of pathogens directly into the blood, as well as protecting the new skin as it grows.

Tears are salty and contain the enzyme lysozyme – both help destroy pathogens

The clotting and healing process that protects the body from the entry of pathogens through an open wound.

Recognising cells

The cell surface membranes are the site of cell identification systems. There are glycoproteins and glycolipids (proteins and lipids with short carbohydrate sections attached to the molecules) and these, along with some membrane proteins, act as antigens identifying one cell to other cells. For example this system enables the cells of the immune system to identify pathogens, cells from other organisms of the same species (eg after an organ transplant), abnormal body cells (eg cancer cells) and toxins produced by pathogens. It is also key in the non-specific responses of the body to invading pathogens.

The white blood cells

Non-specific responses to infection are usually triggered either by body cells breaking down and releasing chemicals, or by pathogens that have been labelled by the specific immune system. Many of the non-specific responses depend on the many different types of leucocytes (white blood cells). The different cells can be recognised both by their appearance and their functions.

The leucocytes or white blood cells play key roles in both the non-specific responses to infection and the specific immune system

Main non-specific responses

These include:

  • Inflammation: this occurs when damage or infection is localised such as bacteria infecting a cut. When connective tissue below the skin and around blood vessels is damaged, mast cells and basophils release chemicals known as histamines. Histamines make the blood vessels in the area dilate, causing a locally raised temperature that reduces the ability of pathogens to reproduce and redness. Histamines also make capillary walls leaky. Plasma containing neutrophils and antibodies from the specific immune response is forced out of the capillaries, causing oedema (swelling) and often pain. The antibodies disable the pathogens, which are then destroyed by macrophages and neutrophils by phagocytosis. The rashes seen in many different infections are a form of inflammation of the skin.

This rash shows typical inflammation.
(Photo credit: CDC)

  • Fevers: during some infections the ‘thermostat’ in our hypothalamus is reset. Normal core body temperature is around 37oC, infection by pathogens can result in a higher body temperature - we have a fever. If an infection is caused by bacteria the temperature rises steadily and remains fairly high until the body recovers or antibiotic treatment is successful. If the pathogen is a virus the temperature tends to ‘spike’. It shoots up high every time viruses burst out of the cells and then drops down again as they reinfect more cells and reproduce again.

A raised temperature helps fight infection in two ways:

  1. many pathogens cannot reproduce effectively at temperatures much above 37oC so their growth is slowed
  2. the specific immune system is more effective at higher temperatures

We monitor temperature carefully when someone is ill because if a fever goes too high it can cause damage or be fatal - over 40°C enzymes start to denature and there can be permanent tissue damage or even death

  • Phagocytosis: this involves white blood cells that engulf and digest pathogens and any other foreign material in the blood and tissues. Phagocytes engulf the pathogen into a vesicle called a phagosome. This fuses with a lysosome and the enzymes break down the pathogen. Phagocytes are very important in both the non specific responses to infection and the specific immune response.

    Neutrophils make up 70% of the leucocytes in the blood, but each neutrophil can only ingest a few pathogens before it dies.

    Macrophages make up about only about 4% of the leucocytes in the blood. They can digest enormous numbers of pathogens because they can renew the digestive enzymes in their lysosomes, which neutrophils cannot do. Cells called monocytes move to infected tissues and become macrophages, so there are lots of macrophages in the tissues where they are needed.

    The pus that sometimes forms in spots or infected cuts is a build-up of dead cells, mainly neutrophils. The dead neutrophils give the pus its yellowish-white colour.