Our immune system cannot be turned on or off like a light switch. On the contrary, it responds to an invasion of bacteria, viruses or parasites through a combination of defensive weapons which adapt smoothly to the situation.
The simplest reaction of the immune system is for example that triggered by a mosquito bite: an inflammation. When the insect has bitten, the immune system sends cell groups that do not have any special defensive skill. These cells, mostly neutrophils, travel along the blood flow tracking non-familiar chemical trails. When they find them, they release chemical signals asking for new forces, that agglomerate
to kill the invader. This is like an infantry attack.
If the invaders are too powerful for this first line of defense, the immune system sends a second wave of cells, the innate immune cells. These cells are programmed with biochemical weapons that can attack a specific type of invaders, including the most common viruses, like cold or gripe viruses.
Sometimes, this is not enough and sometimes the immune system appeals to the most specialized cells of the immune system that learn from experience. Once exposed to a virus or bacteria, they recognize it if it appears again. That's why when you have already had smallpox, it's so difficult getting again.
This third type is made of two parts. The first implies antibodies, molecules produced to stick, like a key into a lock, in the proteins that cover the surface of the viruses or bacteria. The more "keys" there are to bear the ring of the immune cells, the higher the likelihood that the immune lymphocyte B cells stick onto a pathogen, destroying it. After detecting antigens in the blood, some B cells retreat to the lymph nodules, where they turn into plasma cells that synthesize antibodies that can combine to the antigens.
But sometimes the viruses and bacteria use biochemical tramps, disguising themselves in a protein called antigen and avoiding this way the antibodies. In this case, the immune system employs the dendritic cells, which grasp on the invaders and strip in pieces their chemical disguise, exposing them to the attack of the lymphocyte T cells, which engulf and digest them.
The cytotoxic T cells grasp the pathogen and provide it with a toxic lethal dose. After that they separate and the T cells attack other pathogens till they die. The helper T cells release cytokines, hormones that turn on the immune system, which quickly sends more specialized cells for digesting the microbes.
The last regiment is that of the killer cells, non-specialized cells, which do not require the identification of the pathogen and will attack any hostile microbe.
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