In this lesson we'll be learning about the specific immune system, which is the third line of defense in the body against pathogens. In this lesson, we'll explore what cells are involved and how they do their jobs.
What Is the Immune System?
Think of your body as a fortress. Inside, precious cells work day and night to keep you alive. Outside the castle gates lurk dangers, like bacteria and parasites, waiting to come in and take advantage of your resources. The castle walls keep most invaders out, but some sneak through. The ones that penetrate the outer walls might get caught in the moat. However, in the unlikely event something makes it through the outer defense system, there are soldiers on guard ready to fight.
These systems are analogous to the immune system in your body. Your immune system is a collection of cells and tissues that defend your body against invaders. Like the outer walls and moat, your body has non-specific defenses that protect your body against all invaders, including your skin, certain chemicals, and cells that are on guard patrolling your body.
If pathogens do get through, your body activates the third line of defense, which is your specific immune system. Your specific immune system is made of white blood cells called lymphocytes: B-cells and T-cells. These cells recognize the pathogen invading specifically, compared to the second line of defense, which is activated by any pathogen. Your lymphocytes mount a defense system that is catered to that particular invader.
When a pathogen comes in contact with your body, it has to breach the first line of defense to get inside. Your skin and mucus membranes are the main barrier here. Mucus traps the pathogens, and then is forced out of your body when you cough or blow your nose. Your skin also secretes chemicals that have antiviral properties, killing viruses on contact. If the pathogens get through that defense, the next line is non-specific immunity cells that patrol your tissues engulfing pathogens. There are other cells that do this, like macrophages, but the dendritic cells are most important for activating the third line of defense in your body.
Dendritic cells reside in your tissues, waiting for an invader to arrive. When they do find one, they engulf it and digest it. After they do this, they select pieces of the invader called antigens and put them on their surfaces. The dendritic cells migrate back to lymph nodes, key locations in your body filled with immune cells. There, they show the antigens, called antigen presentation, to two types of lymphocytes, T-cells and B-cells, activating them for a full immune response.
Your body has many T-cells, which circulate through your lymphatic system. Each T-cell is unique and only will recognize one particular antigen. Therefore, when the dendritic cell shows the antigen to lymphocytes in the lymph nodes, it is really waiting for the right T-cell to come along, which is fit specifically to the antigen.
Although this may seem inefficient, that T-cell will be a perfect fit for the pathogen, allowing for a strong immune response. Imagine you are the boss of a big company. You can have thousands of workers. Would you rather have thousands of workers that are okay at everything, or a thousand different experts, ready to crush their daily tasks? I would rather have the experts, and your immune system is no different.
There are two types of T-cells, helper T-cells and cytotoxic T-cells. When either T-cell is a match to the antigen presented by the dendritic cell, it divides rapidly in a process called clonal expansion. Helper T-cells, like the name sounds, go on to help activate the rest of the immune system. The helper T-cell secretes chemical messages called cytokines, which recruit other cells, like macrophages, B-cells, and T-cells to mount an attack.
Cytotoxic T-cells, on the other hand, play a more active role in the fight. When activated by the dendritic cells, they also go through clonal expansion. But instead of activating other cells, the main job of the cytotoxic T-cell is to attack infected host cells, like an assassin. Although it may seem dangerous to kill off our own cells, the cytotoxic T-cells do it for the greater good. If the infected cells don't die in a controlled way, eventually the pathogen will kill them anyway, letting virus particles explode out and infect other cells.
The last type of lymphocyte are B-cells. These cells make the antibodies we are so familiar with. B-cells are recruited by helper T-cells and are also activated by dendritic cells. When they are activated, these cells go through clonal expansion and change into plasma cells, which create antibodies.
Unlike T-cells, which act locally, the antibodies are secreted into the blood and travel all over the body. Each antibody is specific for only one antigen. When the antibodies come in contact with an antigen on a pathogen, they stick to it like glue. The 'glue' slows down the pathogen and allows macrophages to engulf it more easily. Since the outside of the bacteria is covered in antibodies, this process also prevents the pathogen from infecting other cells. Some antibodies are also directly attached to the B-cell on the outer surface, helping it find pathogens directly.
Both B- and T-cells form a special type of cell called a memory cell after the infection has passed. These cells stick around after the majority of the white blood cells that were made die off. When the same pathogen is encountered again, these cells are immediately activated because they are a match for the antigens. This is what causes immunity. Vaccines actually inject small antigens or inactive pathogens into the body to create this exact response.
The immune system is a collection of cells and tissues that protects your body from pathogens. These are three lines of defense, the first being outer barriers like skin, the second being non-specific immune cells like macrophages and dendritic cells, and the third line of defense being the specific immune system made of lymphocytes like B- and T-cells, which are activated mostly by dendritic cells, which activate the third line of defense within your body. Helper T-cells activate other cells in the immune system, and cytotoxic T-cells kill infected cells by looking for antigens, or pieces of the invading pathogen. Once B-cells are activated, they become plasma cells, secreting large amounts of antibodies to kill pathogens. B- and T-cells also form memory cells that stick around after the infection has passed. These cells quickly become activated when the same pathogen is encountered again, creating immunity.