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Recognizing the Alien
Jul 1, 2016

One of the most important preventative measures the human body can take against diseases is recognizing harmful things that have entered the body – and recognizing them as soon as possible. After recognizing the enemy, what follows next is deciding what kind of strategy to pursue against it. The bodily defense systems which carry out this task can be separated into two: the innately granted immune system and the one granted later on, known as the adaptive immune system.

The elements of the innate immune system are dendritic cells, macrophages, neutrophils, mast cells, eosinophils, basophils, and natural killer cells. The adaptive immune system has two basic elements, the B and T cells.

The human body rarely has serious reactions against antigens which are not harmful. However, the body responds to harmful substances with the bodily defense system. We can think of the situation like this: imagine that a man visits a certain country and enjoys himself. If he begins to cause harm to that country and its citizens, he will be arrested by the security forces of that country. The human body reacts similarly to unwanted visitors. Invasive substances that cause a defensive response are called immunogens. Disease causing germs are made up of structures such as fats, carbohydrates, proteins, DNA, and RNA. As cells of the immune system attack the organic structures of germs, the strongest reactions happen against the proteins of complex structures. The question is: how does the body recognize these harmful molecules and organic structures?

Different elements of the immune system recognize pathogens through different ways and means. The different elements are equipped with specific receptors, which enable them to work together as a defensive unit. These receptors can be compared to locks, alarms, and other controls used to protect a house. They make it possible to alert the body to most unwelcome visitors. The innately granted immune system has many different ways of recognizing aliens. These include the killer-activation and killer-inhibition receptors, the complementary system, and Fc receptors.

Natural receptors of the immunity system

People are born equipped with the innately granted immune mechanisms. These loyal and dutiful elements quickly recognize and respond to pathogens. If we continue thinking of the structures formed by the immune system as a key and lock, then the lock can be defined as the receptors of the innate defense system. Once this mechanism is ready, different signals given to the core of the cell trigger the release of certain molecules (cytokine and chemokine). Similar to cannonballs punching holes in castle walls, these molecules upset the membranes of germs. Thus the process of eliminating harmful pathogens begins. If it is not possible to handle the situation in a short time, the defensive cells call other elements of the defense system to help. Sometimes, natural killer cells and eosinophils take charge. The defensive line works in total cooperation. The key and lock relation constitutes the beginning of this defensive line.

Toll-like receptors won the Nobel Prize

Toll-like receptors have an interesting story. A German scientist spotted a certain gene found in drosophila (the vinegar fly) and named it “toll” (meaning great, fantastic, or stunning in German). Later on, since the structure and placement of nucleotides are similar to the “toll” gene, the newly discovered immune system receptors were named toll-like receptors. The scientists who made this discovery won the 1995 Noble Prize in medicine.

These receptors, found in the membranes of defensive cells, are responsible for recognizing and capturing germs. There are 13 types of defined TLRs in the human defense system. While some TLRs (TLR 3, 7, 8, and 9) are found inside the cells and arrest the harmful intruders, some are found in the cell walls and function like guards patrolling a border. By means of these guards in the cell walls, germs are spotted so other elements can eliminate them. These dutiful guards fulfill the duty they are created for – and they do it in a wonderful fashion.

Killer-activated receptors

Harmful intruders, such as viruses and parasites, sometimes invade the body’s cells. Some germs continue their lives by using the genetic material, metabolism and energy of the cells they invade. Moreover, they continue their expansion to healthy cells. If they expand enough, they can threaten the entire body.

As part of the body’s defense system, molecules signifying stress appear on the surfaces of these invaded cells, a sort of “warning” that the cell is occupied. In such a case, the killer-activated receptors recognize the cells with stress molecules on their surfaces and connect to them. Then a necessary sacrifice is made: the “occupied” cells are eliminated to prevent the germs from expanding to the rest of the body.

Killer-inhibitor receptors

All cells with a nucleus possess the tissue conformity complex known as MHC-I, which can be compared to an ID card, which shows a certain cell to be a healthy piece of the body. When some cells are invaded by germs, or lose their qualities due to reasons like cancer, they cannot show their IDs anymore. Namely, MHC-I is either too weak or is not seen on the cell’s surface. Killer-activated receptors, which are constantly on patrol, demand every kind of cell to show its ID in order to verify there are no alien invasions. The MHC-I identity of a cell is recognized and controlled by the killing-inhibitor receptors—or the bar code readers—of the natural killer cells. At this control, the cells that fail to show their ID card are accepted as invaded or as having contracted cancer. The cells in such condition are eliminated by the natural killer cells for the health and continuity of the entire body.

Complement system receptors

After entering the body, many viruses can reach different parts of the body by joining the bloodstream. In addition, blood also contains the harmful substances of any disease-causing elements dissolved in it. In order to recognize these harmful elements and toxic substances, a special defense team (the complement system) is assigned to the task. Many molecules in the protein structure take part in the complement system. By means of this system, the body recognizes and eliminates the potential threats which pass through the body’s physical, chemical, and biological barriers, and which might reach all the body’s cells via the blood stream.

This defense system is an interconnected and cooperative one. In situations like the deficiency of the elements that make up this system, the body is not properly defended and suffers from different diseases. The pioneering element of the complement defense system, known as C3b, binds to the lipopolysaccharide molecules on the membranes of microorganisms and triggers the complement system’s reaction. As a result of this reaction, the other elements of the defense team (C6, C7, C8, and C5b) are activated and embedded in the cell membrane of the microorganisms. By opening holes in the membrane, they cause the microorganism to die.

The design of our immune system shows how perfectly equipped humanity is for life on this planet. Though we consciously protect our homes and property, we never think about what is happening inside our bodies. If somehow we were supposed to consciously take care of these defensive processes, we would never be able to have “inner peace.” Thankfully, we are not alone.