The immune system

The immune system is responsible for protecting our body from other pathogenic microorganisms such as viruses, bacteria, protozoa, fungi or the molecules produced by them.

The immunity is the resistance state of the natural or acquired from an organism to a specific disease. The immune response can be nonspecific and specific.

Nonspecific immunity

This type of nonspecific immune response includes those barriers that try to prevent the entry of pathogenic microorganisms, without focusing on any specific type. There are two types:

• External defenses.
• Internal defenses.

External defenses: the primary barriers

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.

By Alejandro Porto [CC BY-SA 3.0], via Wikimedia Commons

Activity: Routes of contagion.

Internal defenses: secondary barriers

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.

By Phagocytosis2.png: Graham Colm Talk.GrahamColm at en.wikipediaderivative work: Retama (Phagocytosis2.png) [CC BY-SA 3.0 or GFDL], from Wikimedia Commons

Activity: Phagocytosis.

• 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.

Specific immunity

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.

If the microorganism changes, such as the flu virus, the memory lymphocytes are not able to detect them and the person suffers from the disease again.

Activity: History of an infection.

Webquest: The curious case of chickenpox on the Simpsons.