The Immune System – Overview
We are surrounded by billions of bacteria and viruses. To many of them, a human being is like a walking smorgasbord, offering nearly limitless resources that they can use for energy and reproduction. Luckily for us, getting into the human body is not an easy task! From the point of view of these tiny organisms, a human is a bit like a fortress. The skin is thick and very hard to penetrate. In addition, the skin also produces a variety of substances that are harmful to invaders. Openings such as the eyes, nose, and mouth are protected by fluids or sticky mucus that capture harmful attackers.
The respiratory tract also has mechanical defenses in the form of cilia, tiny hairs that remove particles. Intruders that get as far as the stomach are up against a sea of stomach acid that kills most of them. But in spite of our fantastic defenses, hostile invaders still manage to get through. Some enter along with our food, while others may sneak in via the nose. And, as we all know, many things can break through our skin. In everyday life we often receive cuts or scrapes, and every time this happens we face the risk of a full-scale invasion from bacteria or viruses.
What is the magic, then, that keeps us healthy most of the time? When we receive a cut, and when invaders enter the body, cells are destroyed. The dying cells trigger an automatic response called inflammation, which includes dilated blood vessels and increased blood flow. An inflammation is the body’s equivalent to a burglar alarm. Once it goes off, it draws defensive cells to the damaged area in great numbers. Increased blood flow helps defensive cells reach the place where they’re needed. It also accounts for the redness and swelling that occur.
Immune Cells: The Defense The defensive cells are more commonly known as immune cells. They are part of a highly effective defense force called the immune system. The cells of the immune system work together with different proteins to seek out and destroy anything foreign or dangerous that enters our body. It takes some time for the immune cells to be activated – but once they’re operating at full strength, there are very few hostile organisms that stand a chance. Immune cells are white blood cells produced in huge quantities in the bone marrow.
There are a wide variety of immune cells, each with its own strengths and weaknesses. Some seek out and devour invading organisms, while others destroy infected or mutated body cells. Yet another type has the ability to release special proteins called antibodies that mark intruders for destruction by other cells. But the really cool thing about the immune system is that it has the ability to “remember” enemies that it has fought in the past. If the immune system detects a “registered” invader, it will strike much more quickly and more fiercely against it.
As a result, an invader that tries to attack the body a second time will most likely be wiped out before there are any symptoms of disease. When this happens, we say that the body has become immune. Bacteria and Viruses: Our Main Enemies | A virus needs a host cell to reproduce. | Now that you know a bit about our defenses, let’s take a closer look at our primary enemies. Bacteria and viruses are the organisms most often responsible for attacking our bodies. Most bacteria are free living, while others live in or on other organisms, including humans.
Unfortunately, many bacteria that have human hosts produce toxins (poisons) that damage the body. Not all bacteria are harmful, though. Some are neutral and many are even desirable as they fulfill important functions in the body. Bacteria are complete organisms that reproduce by cell division. Viruses, on the other hand, cannot reproduce on their own. They need a host cell. They hijack body cells of humans or other species, and trick them into producing new viruses that can then invade other cells. Frequently, the host cell is destroyed during the process. Pathogens and Antigens
In daily life we might speak of viruses, bacteria, and toxins. However, when reading about the immune system you’ll often come across the words antigen and pathogen. An antigen is a foreign substance that triggers a reaction from the immune system. Antigens are often found on the surfaces of bacteria and viruses. A pathogen is a microscopic organism that causes sickness. Hostile bacteria and viruses are examples of pathogens The Immune System – in More Detail The immune system is one of nature’s more fascinating inventions. With ease, it protects us against billions of bacteria, viruses, and other parasites.
Most of us never reflect upon the fact that while we hang out with our friends, watch TV, or go to school, inside our bodies, our immune system is constantly on the alert, attacking at the first sign of an invasion by harmful organisms. The immune system is very complex. It’s made up of several types of cells and proteins that have different jobs to do in fighting foreign invaders. In this section, we’ll take a look at the parts of the immune system in some detail. If you’re reading about the immune system for the first time, we recommend that you take a look at the Immune System Overview first (see link below).
The Complement System The first part of the immune system that meets invaders such as bacteria is a group of proteins called the complement system. These proteins flow freely in the blood and can quickly reach the site of an invasion where they can react directly with antigens – molecules that the body recognizes as foreign substances. When activated, the complement proteins can | -| | trigger inflammation| | -| | attract eater cells such as macrophages to the area| | -| | coat intruders so that eater cells are more likely to devour them| | -| | kill intruders|
Phagocytes This is a group of immune cells specialized in finding and “eating” bacteria, viruses, and dead or injured body cells. There are three main types, the granulocyte, the macrophage, and the dendritic cell. | The granulocytes often take the first stand during an infection. They attack any invaders in large numbers, and “eat” until they die. The pus in an infected wound consists chiefly of dead granulocytes. A small part of the granulocyte community is specialized in attacking larger parasites such as worms. | The macrophages (“big eaters”) are slower to respond to invaders than the granulocytes, but they are larger, live longer, and have far greater capacities. Macrophages also play a key part in alerting the rest of the immune system of invaders. Macrophages start out as white blood cells called monocytes. Monocytes that leave the blood stream turn into macrophages. | | The dendritic cells are “eater” cells and devour intruders, like the granulocytes and the macrophages. And like the macrophages, the dendritic cells help with the activation of the rest of the immune system.
They are also capable of filtering body fluids to clear them of foreign organisms and particles. | Lymphocytes – T cells and B cells | | The lymphatic system| The receptors match only one specific antigen. | White blood cells called lymphocytes originate in the bone marrow but migrate to parts of the lymphatic system such as the lymph nodes, spleen, and thymus. There are two main types of lymphatic cells, T cells and B cells. The lymphatic system also involves a transportation system – lymph vessels – for transportation and storage of lymphocyte cells within the body.
The lymphatic system feeds cells into the body and filters out dead cells and invading organisms such as bacteria. On the surface of each lymphatic cell are receptors that enable them to recognize foreign substances. These receptors are very specialized – each can match only one specific antigen. To understand the receptors, think of a hand that can only grab one specific item. Imagine that your hands could only pick up apples. You would be a true apple-picking champion – but you wouldn’t be able to pick up anything else. In your body, each single receptor equals a hand in search of its “apple. The lymphocyte cells travel through your body until they find an antigen of the right size and shape to match their specific receptors. It might seem limiting that the receptors of each lymphocyte cell can only match one specific type of antigen, but the body makes up for this by producing so many different lymphocyte cells that the immune system can recognize nearly all invaders.
T cells T cells come in two different types, helper cells and killer cells. They are named T cells after the thymus, an organ situated under the breastbone. T cells are produced in the bone marrow and later move to the thymus where they mature. Helper T cells are the major driving force and the main regulators of the immune defense. Their primary task is to activate B cells and killer T cells. However, the helper T cells themselves must be activated. This happens when a macrophage or dendritic cell, which has eaten an invader, travels to the nearest lymph node to present information about the captured pathogen. The phagocyte displays an antigen fragment from the invader on its own surface, a process called antigen presentation. When the receptor of a helper T cell recognizes the antigen, the T cell is activated.
Once activated, helper T cells start to divide and to produce proteins that activate B and T cells as well as other immune cells. | PARTS AND FUNCTIONS White Blood Cells * The smallest parts of the immune system are the myriad types of white blood cells that are responsible for demolishing malicious bacterial, viral and tumor cells. T cells serve as both managers and infection killers. They are responsible for activating and communicating with other types of white blood cells before destroying malignant cells like parasites and tumors. Natural killer cells directly attack virus cells and tumor cells such as lymphoma, melanoma and herpes.
They work alone without communicating with other parts of the immune system. B cells work to produce antibodies that attach themselves to foreign cells as a sign to natural killer cells and T cells to attack and destroy. Bone Marrow * An essential aspect of the immune system–and the origin of all types of immune system cells–is red bone marrow. Bone marrow is a specific type of tissue that grows in the empty centers of bones. This tissue uses the process of hematopoiesis to manipulate its own stem cells into B cells and natural killer cells, as well as the foundational pieces of other immune ells like T cells. Once they are created, these cells migrate out of the marrow tissue and circulate through the blood stream to infection sites, other glands or around the body as general patrol entities. Thymus Gland
* The foundations of T cells produced in the bone marrow, called thymocytes, leave the tissue and travel to the thymus gland for completion. The thymus is a small gland located near the lungs in the upper torso. Thymocytes complete their maturation in the thymus through the process of thymic education, where each cell is developed and examined for maximum efficiency.
Cells that are not strong enough to provide immune support are destroyed and absorbed by the thymus, while the successfully matured cells are excreted from the gland into the blood stream. Spleen The spleen, which is located on the left side of the abdomen just under the lung, is a blood filter that works to remove malignant cells from the blood stream. To assist in this function, it holds a significant store of B cells, T cells and natural killer cells to help eliminate any contaminants that are caught.
The spleen also assists in immune function by holding a store of red blood cells and platelets that can be deployed as support for the immune cells in the event of an infection or wound. Lymph Nodes * Lymph nodes, found throughout the body, are also integral parts of the immune system that filter tissue fluid for bacteria cells, tumor cells and viral particles. Like the spleen, lymph nodes are full of the various types of white blood cells that clean the lymph fluid before returning it to various areas of the body.
Lymph nodes are located in the head, neck, arms, legs, abdomen and genital area of the human body and are connected through a network of afferent lymphatic vessels. In the event of an infection, white blood cells can use these lymphatic vessels to quickly communicate with other parts of the immune system. SKIN The skin is the largest organ in area. With the Langerhans cells in the lowest epidermal layers, it is equipped with specialized immunologically competent cells. The Langerhans cells play a central role in the skin’s immune system and are an integral part of the body’s total defence system.
The body’s own defence against microorganisms begins directly at the skin surface. Special fatty acids from the sebaceous glands (i) and the secretions of certain bacteria belonging to the physiological skin flora inhibit the growth of fungi and bacteria. Certain enzymes present in sweat (lysozymes) can destroy the cell walls of invading bacteria. If a foreign body passes this first line of defence – for example, due to skin damage – the skin’s immune system responds. Many cells help fend off foreign bodies. Among these are cells – like the Langerhans cells – that are specific to the skin’s immune system.