A series of barriers act nonspecifically against any pathogenic microorganism that tries to infect the person. They can be of three types:
- Physical. The skin and mucous membranes are the first defensive structures that a microorganism has to cross. They can only go through the skin taking advantage of a broken wound. The mucous membranes, which line the natural orifices of the body, are more accessible routes, which is why they are protected with antimicrobial secretions.
- Chemicals. Substances that prevent the entry of microorganisms. For example, tears contain lysozyme that prevents the growth of bacteria, the respiratory tract has mucus that retains solid particles that enter with the air, the stomach and vagina have a lot of acidity to prevent the entry of microorganisms, the sebaceous glands produce a substance that together with sweat and dead cells form a protective acid-fat layer, etc.
- Biological. There are microorganisms that live in symbiosis in our body, benefiting us and compete with other pathogenic microorganisms that are harmful.
Activity: Routes of contagion.
If the microorganisms manage to penetrate the body, a second barrier, also nonspecific, will try to prevent infection.
The defense response to this invader can be of two types, mainly:
- Phagocytosis. Made by phagocytes, a type of white blood cell. Phagocytes are cells that "engulf" foreign particles (microbes, dead cells, cancer cells, etc.) that enter the body. They introduce them as if it were their food in a vacuole (phagocytic vacuole), encompassing them by pseudopods. They destroy them by digesting them with the help of digestive enzymes.
- The inflammatory reaction. When microorganisms enter, vasodilation occurs, dilating the capillaries so that more blood can arrive with phagocytes to fight them, which causes redness and an increase in temperature in that area. Thus, phagocytes act to prevent microorganisms from spreading throughout the body.
If the pathogenic microorganisms overcome the nonspecific barriers, the specific immunity, exclusive for each type of invader, comes into operation. Another type of white blood cell, lymphocytes, are responsible for this other type of defense mechanism.
The lymphocytes act a specific way, as they are able to recognize antigens (substances recognized as foreign) and combat. The main types of lymphocytes are:
- T lymphocytes. They recognize and destroy antigens on infected cells.
- B lymphocytes. They produce proteins, antibodies, that act as a defense against a foreign substance, antigens. Each antibody is specific for each antigen.
When the body has fought against a pathogenic microorganism, some lymphocytes were activated and remain in the body throughout life, they are memory lymphocytes. If the same microorganism tries to produce a new infection, it will quickly be detected by the memory lymphocytes and they will be neutralized. The person will not suffer from this disease again because they are already immunized, thanks to memory lymphocytes, against this pathogenic microorganism.
Activity: History of an infection.
12.5.- What is the difference between an antibody and an antigen?
12.6.- Explain the difference between the response that make phagocytes that makes lymphocytes when bacteria enter our body.
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