Our environment has billions of pathogenic microorganisms – bacteria, fungi, viruses and parasites –relentlessly trying to enter our body and infect us. And why wouldn’t they! Our body not only provides a very conducive environment for them to grow and multiply, but where they also release toxins and cause infections ranging from a common cold to measles, dysentery, tuberculosis, chicken pox, malaria and other serious situations. Fortunately, it is not as easy as it sounds, thanks to the defense mechanisms installed by the immune system – the very elaborate and powerful anti-virus software of our body.
Our immune system is a complex network of cells, tissues and organs that continuously work in tandem to protect the body against any microbial invasion, and identify and destroy the pathogens if they are still able to invade. And what’s more, it is even able to remember the invader so as to launch a much faster attack if same microbe strikes again, making our bodies immune against it.
How is our immune system able to perform this amazing feat? The workings of the immune system are not entirely clear and the subject is a hotbed of research with many mysteries remaining to be unfolded. But as a simple explanation, we can say that the immune system employs more than one mechanism to keep us safe from invading pathogens, infections and diseases. To understand this, let us quickly go over the concept of innate immunity and adaptive immunity – the two arms of the immune system.
Innate and Adaptive Immune System
Some components of the immune system, white blood cells called macrophages, are on a constant patrol looking out to find germs and destroy them as soon as they enter the body. This process gives us natural immunity also called innate or inborn immunity. But what if this first line of defense fails in its function? That is when our adaptive, or acquired immunity kicks in – employing more fierce and powerful T lymphocytes and B lymphocytes cells from its arsenal. Let us figure this out in more details.
The innate immune system comprises of cells and proteins that are always in alert mode and ready to kill and engulf pathogens at sight without any discrimination. The main weapons used in this fight are physical barriers like skin and the respiratory tract, followed by a wide variety of immune cells such as phagocytic leukocytes, dendritic cells, and Natural Killer cells that work with numerous different proteins to seek and destroy the enemy. Our innate immunity is a non-specific attack against any pathogen, triggered within a few hours of the arrival of antigen in our body. The chemical properties of the foreign antigens are responsible for this trigger. But sometimes this mechanism is not enough to contain the pathogens. Now we enter the realm of adaptive immunity.
The adaptive immune system employs even more highly specialized immune cells to rid the pathogens. The components of adaptive immunity are usually silent but, when activated, launch a ferocious attack by activating other immune cells and releasing special proteins to neutralize the microbes. Adaptive immunity also plays an important role in creating immunological memory – enabling the immune system to remember any pathogen it has encountered and fought before. This memory allows the immune system to respond much more quickly and efficiently the next time it faces the attack.
It sounds like some great modus operandi. Our immune system works by identifying, recognizing and destroying foreign agents. At the same time, this amazing machinery also knows how not to attack its own cells. Considering we have both good and bad bacteria residing in our body, how is our immune system able to recognize an invading pathogen? How is our immune system able to differentiate between the body’s own cells and those of the harmful foreign micro-organisms? What gives our immune system capacity to recognize cellular components of its own body? How does it all work?
How does our immune system recognize foreign invaders?
The mystery lies in the proteins and sugars, called antigens, present on the cellular surface of almost all living cells and viruses. These chemicals are unique to all living beings. For example, the proteins on a bacteria or a virus will be different to the proteins on the cells in our body. True, our body’s cells also have surface proteins but scientists suggest that our immune system has been exposed to its own molecules, cellular proteins and sugars in the womb. Consequently, our immune system has learned at a very early stage to recognize these proteins as ‘self’ and ignore them as harmless, failing which it can even attack its own cells and tissues resulting in an unfortunate and less-understood condition called auto-immune disorder.
Antigens: An antigen is any substance that triggers the immune system to generate anti-bodies. If you split the word, antigen literally means antibody-generator.
Antibodies: Specialized white blood cells, called B cells, produce y shaped proteins called antibodies. These proteins are able to chemically fit to a specific antigen and attach to it, and help to neutralize the pathogens in a number of ways.
B and T cells: Identifying, killing and memorizing a pathogen
All the talk about antigens and antibodies will be incomplete without mentioning two types of lymphocytes – B and T cells. These highly specialized white blood cells are defender cells tailor-made to counter a specific germ.
- Each lymphatic cell has receptors on its surface that helps it to recognize foreign protein markers or antigens on the pathogens.
- These receptors are highly evolved and each receptor is able to match only one specific antigen. For example, when a certain germ infects our body, only the T and B cells that have receptors to match or fit this particular antigen are activated.
- These selected T and B cells will quickly reproduce and create a powerful army to fight and contain the infection.
- Some T and B cells are assigned to remember the invader.
If the receptors of each lymphocyte cell are able to fit only one specific type of antigen, does this make the process limiting? Well, we don’t call our immune system a marvelous invention of nature without a reason. Our immune system can produce different lymphocyte cells that can fit almost every possible shape of antigens.
T Cells Functions
T cells originate in the bone marrow and migrate to the thymus, where they mature. There are two types of T cells; helper T cells and Killer T cells. The main function of the helper T cells is to release proteins to activate Killer T cells and B cells, while Killer T cells specialize in destroying cells infected by viruses and bacteria. Killer T cells have the capability to kill cancerous cells too.
B Cells Functions
When a foreign micro-organism invades the body, B cells recognize it and bind to the antigen present on its surface. The proteins produced by helper T cells further kick the B cells into action and that’s when B cells quickly divide into two new types of cells – plasma cells and B memory cells. Plasma cells make Y shaped antibodies that circulate and seek a matching antigen and binds to it. On meeting and binding to its particular antigen, plasma cells replicate quickly and make many copies of antibodies in the process. Antibodies can immobilize bacteria, create an environment to encourage other immune cells to gobble up the pathogen, and release proteins to activate other immune cells to help in the battle. Antibodies also neutralize toxins from the pathogens and render viruses useless to infect new cells.
Some of the B and T cells remain as memory cells – helping the immune system to act quickly and launch a fiercer attack in case the same pathogen (antigen) attacks the body again. This time, the immune system will be able to wipe out these pathogens even before you experience any telltale signs of infection or inflammation.
Needless to say, our immune system works in a very complex environment using a wide array of immune cells. These cells, when activated, undergo strategic transformations and produce potent chemicals performing a range of functions to launch immune-modulatory responses – such as signal and enlist the fellow immune cells in the fight; mobilize macrophages, natural killer cells and other proteins to the infection site; inhibit pathogen duplication; and even activate genes that express anti-viral proteins, thereby defending nearby cells from getting infected and preventing the infection to spread further. This extremely elaborate and self-motivated communication network works in many known and unknown ways to keep us safe and healthy.