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Warzone: S. aureus bacteria escaping destruction by human white blood cells
Image: NIAID/RML

White blood cells are at once the intelligence agents and the crack commandos of the body’s immune system. They’re the special forces that hasten to the scene of an attack whenever germs or infections are entering the body. Their mission: to take out invading pathogens, and generally to defend against infectious diseases and foreign substances – like potentially malevolent bacteria.

White knight: Electron microscopic image of a single human Lymphocyte cell

Image: Unknown photographer/artist

There are five main types of white blood cells – all constantly on the lookout for signs of disease and armed with various ways in which they can attack – but by far the most prolific are the Neutrophils and the Lymphocytes, which together make up between 87 and 94% of the white blood cells in our body. Found mainly in the lymphatic system, Lymphocytes step up in increasing numbers in response to viral infections and tumour cells. Lymphocytes detect the foreign substances of germs in the body and produce protective antibodies and cells specifically designed to target and overpower them.

Mid-fight: Neutrophil immune cell engulfing anthrax bacteria

Image via Etxrge

Big shots in the body’s fight against bacterial infections, Neutrophils circulate in the bloodstream, moving into infected tissue to attack the bacteria. Neutrophils take a more direct, ‘hands-on’ approach than Lymphocytes: after responding to chemicals released by bacteria and dead tissue cells, they rush towards the area of highest concentration, then surround and devour the offending cells, destroying them with powerful enzymes. Called phagocytosis, this vacuum cleaner-like process uses up so much energy that the Neutrophils die soon after, and their activity and death in large numbers forms pus.

Catch the bacteria: Neutrophil cell chases and devours Staphylococcus aureus

While the majority of bacteria in the body are rendered harmless by the defensive efforts of the immune system, and indeed a few are beneficial, certain species of bacteria are pathogenic and a major cause of disease and death. The line-up of infections caused includes tetanus, typhoid fever, diphtheria, syphilis, cholera, food poisoning, anthrax, leprosy and bubonic plague. The most widespread fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people every year, mainly in sub-Saharan Africa, according to the World Health Organisation.

Engulf war: Neutrophil’s phagocytosis of Streptococcus pyogenes bacteria

Image: Copyright © PNAS 1993 in article by Kobayashi et al

Each species of pathogenic bacteria has a range of characteristic interactions with its human hosts. So for example, a bacterial organism like Streptococcus can cause skin infections, pneumonia, meningitis, and even necrotizing fasciitis, the infamous – though rare – “flesh eating” infections – the subject of much sensationalist coverage in the media. However, the irony is that it is normal for Streptococci to exist in the skin, mouth, intestine and upper respiratory tract of humans without causing any disease at all, and they are even a basic ingredient in Emmental, the famous Swiss cheese.

Soon to be dead heroes: White blood cell attacking an infection

Image via Life with Father

Suffice to say that although, even among pathogenic strains, not all bacteria are bad bacteria, some organisms do overwhelm our body’s resistance, proving too much even for the specialist, bacteria-killing white blood cells to cope with. A higher than average white blood cell count (WBC) due to the body’s increased production of the all-action Neutrophils is the sign of a serious bacterial infection, while when the WBC is low, there may not be enough Neutrophils to defend against attack. Faced with certain strains, sometimes it’s the white blood cells that find themselves engulfed.

Sources: 1, 2, 3, 4, 5, 6, 7, 8

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