Spores of fungus, Candida albicans

The long strands are the tubular filaments (hyphae) that have developed from the fungal spores. Yeast cells (rounded, yellow) are budding from the ends of the hyphae (red). Candida albicans causes the infection known as candidiasis which affects the moist mucous membranes of the body, such as skin folds, mouth, respiratory tract and vagina. Oral and vaginal conditions are known as thrush.

More about fungi

Immune system

An infection can be seen as a battle between the invading pathogens and the host. Our bodies are equipped to fight off invading microbes that may cause disease. These are called our natural defences.

  1. A macrophage

    A macrophage (type of phagocyte) engulfing a bacterium.

    A macrophage (brown) which is a type of phagocyte engulfing a bacterium (blue). The nucleus of the macrophage is red.

  2. A blood clot

    © CNRI / Science Photo Library

  3. Computer artwork of bacteria

    Computer artwork of bacteria (blue and green) on human skin.

  4. Staphylococcus aureus

    Staphylococcus aureus bacteria (yellow) sticking to the mucus (blue) on the hair-like cilia.

  5. Macrophage

    Macrophage (yellow) engulfing Escherichia coli bacteria (pink rods).

    This process is called phagocytosis.

An infection can be seen as a battle between the invading pathogens and the host. Our bodies are equipped to fight off invading microbes that may cause disease. These are called our natural defences.

First line of defence

The first line of defence is non–specific and aims to stop microbes from entering the body. The skin and mucous membranes act as a physical barrier preventing penetration by microbes.

If the skin is cut then the blood produces a clot which seals the wound and prevents microbes from entering.

The surfaces of the body – the skin, digestive system, and the lining of the nose – are covered by a community of microbes called the normal body flora. They help to protect a host from becoming infected with more harmful micro-organisms by acting as a physical barrier. The normal body flora colonises these linings which reduces the area available for pathogens to attach to and become established. It also means that the harmful microbes have to compete with the normal body flora for nutrients. The average human gut contains around 1 kg of these good bacteria which is equivalent to one bag of sugar.

The respiratory system – the nose and passageways leading to the lungs – is lined with cells that produce sticky fluid called mucus that traps invading microbes and dust. Tiny hairs called cilia move in a wave-like motion and waft the microbes and dust particles up to the throat, where they are either coughed or sneezed out or swallowed and then passed out of the body in faeces.

The body produces several antimicrobial substances that kill or stop microbes from growing. For example the enzymes in tears and saliva break down bacteria.

The stomach produces acid which destroys many of the microbes that enter the body in food and drink.

Urine as it flows through the urinary system flushes microbes out of the bladder and urethra.

Second line of defence

If microbes do manage to get inside the body then the second line of defence is activated. This is also non-specific as it stops any type of microbe. Phagocytes are a type of white blood cell that move by amoeboid action. They send out pseudopodia which allows them to surround invading microbes and engulf them. Phagocytes release digestive enzymes which break down the trapped microbes before they can do any harm. This process is called phagocytosis.

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