In spite of the excellent first line defense, it is inevitable that some pathogens (disease-causing elements such as bacteria and viruses) can manage to take them through the first line defense system. It could be trough a wound, or that the reproduction of the pathogens is too fast and successful. Then the innate defense system takes over. The white blood cells (leukocytes) attack immediately the invaders, either by isolating and then resolve the pathogens, or by “spraying” them with deadly antibodies. The innate defense system is made up of a) the “fixed” system, or the innate immune system, and b) a changeable and adaptable system, or also called the “adaptive immune system. Both of these systems can attack pathogens. All plants and animals have an innate immune system. That act almost instant and in full force. Through evolution over the last millions of years, it has been developed and adapted to the organism to which it lives in, and it is doing a brilliant job. The innate immune system is made up of white blood cells (leukocytes) that move constantly around in the body hunting for harmful invaders. There are many types of leukocytes, each with their specialized task and behavior. Due to the fact that the innate immune system reacts so quickly, it normally takes out the pathogens before they escalate in numbers, and outnumber the leukocytes.
However, bacteria can double about once hour. After 10 hours it is then 1,000 x so many bacteria that originally, and after 24 hours over 1 million times as many. Then it’s easy to understand how important it is that the leukocytes attack immediately.
But also the bacteria adapts and changes over time, and therefore, nature has developed the adaptive immune system, which to a large extent can adapt to the pathogens that are unknown and would have gone clear of the innate immune system.
Adaptive immune systems can be found only at vertebrates. This system attacks with weapons adapted to the specific intruder, and has, in contrast to the innate system, a memory. This means that when the pathogen first is defeated, a later attack by the same (or similar) type of pathogen result in quicker response and faster defeating of the attack. The affected have become immune and will not be sick, or at least less sick than if it had not hosted the attack before.
Also here, there are specialized leukocytes that do the job. However, it is still a lot we don’t understanding about the behavior of the adaptive immune system, but the next paragraph lists up some of the complexity.
The white blood cells (leukocytes) functions of the immune system:
Leukocytes are a particularly important part of the body’s immune system. They are independent cells that move around through the cardiovascular system, the lymphatic system or body tissue on continuous search of pathogens, i.e. alien organisms (viruses, bacteria, parasites, fungi, etc.) that can provoke diseases. Leukocytes are living only a few days.