The immune system in general is made up of chemicals and special cells that fight infections and threats to our health. What makes up the system largely are the white blood cells, which are made in the bone marrow; the white blood cells, when in action, move through blood and tissues. Any time our body reacts to threats, a germ is defeated, and once this is done, our immune system remembers that microbe; therefore, when our system encounters the same type of microbe once more, it is able to defeat it quickly.
Firstly, I should address what exactly a threat to our bodies would be: pathogens. Pathogens are any living organisms or viruses capable of causing a disease (includes bacteria, protozoa, fungi and worms). Exposure to pathogens doesn't necessarily result in disease but depending on the conditions in which they enter and we are exposed to them there is the possibility. Most that enter our body has developed an immunity to that pathogen. Now a general way that people think of being protected from these threats are antibiotics, chemicals that take advantage of the difference between prokaryotic and eukaryotic cells. One type of antibiotic may selectively block protein synthesis in bacteria while another type may inhibit the production of a new cell wall by bacteria, blocking the ability of the pathogen to grow and divide (spread into an infection). It is important to note that antibiotics are used to treat bacterial diseases and not viral diseases because it blocks specific metabolic pathways and cell wall production in bacteria; the viruses reproduce using the host cell metabolic pathways; Viruses, contrastingly, don't have metabolic pathways, so they aren't affected by the antibiotics.
Now the problem really is what happens when the pathogen successfully enters our bodies. At first leucocytes (white blood cells) help fight off the pathogens and produce immunity for them. Macrophages, which are large white blood cells, can change their shape to surround the invader and take it in through the process of phagocytosis (process of destruction). The macrophages are able to squeeze they way in and out of small blood vessels, and when it meets a cell, it recognizes whether the cell is a natural part of the body ("self") or if its not part of the body ("not-self"). This would be a non-specific response because the identity of the pathogen has not been determined, its only response is that it should be removed.
Antibodies (protein molecules) are then produced in response to a specific/different pathogen. Each type of antibody is different because each type is produced in response to a different pathogen. The difference between a antigens and antibodies are that antigens are substances and molecules that cause antibody formations while antibodies are proteins and molecules that recognize antigens. The production of antibodies starts when the antigen stimulates an immune response. The antigen is then engulfed by macrophages, incorporated to the macrophage membrane, and presented to the T-helper cell (type of white blood cell). Once this is done the helper T-cells bind to the antigens, and when activated the B-cells are also activated by the T-cell. Next, the antibodies are produced by B-cells in response to specific antigens, and the B-cells clone. Following, it goes into the plasma cells and memory cells, where plasma cells produce specific antibodies to the antigen. Consequently, the memory cells are used as a long-term immunity because it acts faster and responds faster later on.
I think that it's important to point out different reactions of defense that our body has to different threats.
Since the skin is opened, microbes are able to enter our body freely. The immune system, therefore, sets up a defense to the invasion. This defense would be that the white blood cells that are in the blood are alarmed chemically by the cells that are damaged. Then, it moves towards the opening (the cut) and starts eating the bacteria (not digesting). So when we see pus in our wounds, it's actually dead microbes along with dead white blood cells. Next, the B-lymphocytes attempt to identify the microbe - if it has already been detected and defeated before, the lymphocytes know what antibodies need to be used to kill the microbe. The T-lymphocytes (a variety of helper cells) help the B-lymphocytes to make the antibodies, if the microbe is not yet known. The killer cells then attack the infection to cure it.
From my understanding, I think that a fever is simply an alarm to us that something is wrong, it's not actually the problem - which would actually be an infection. In a fever, there is a rise in body temperature; this rise in temperature can kill some microbes. A fever may also set off a repair process to the body.
A viral infection:
The immune system keeps what I see as a record of microbes that have entered the body and been defeater. So it can destroy the microbe rapidly if these microbes enter the body again before they multiply or cause any other reactions on our bodies. There are some infection that have to be fought more than once because so many viruses can cause the same disease, like the influenza and the cold. For example, catching a cold from one specific virus doesn't mean that you have immunity against other viruses that give you the cold.
This works like copying/ acting out the body's natural immune response. Immunization is basically referred to vaccines, where a small amount of the specially treated virus, bacterium, or toxin is injected into our bodies. Once this is done, our bodies create antibodies. So the next time that the body of the individual encounters and is exposed to the actual virus, bacterium, or toxin, they're bodies will destroy them and their health won't be affected.
Defenses of our physical body:
Our skin itself is a barrier of infection once it is intact and protects pathogens to enter living tissues. The acidity and oiliness of our skin prevents the entering of pathogens as well. Also, the acidic environment of our stomach helps kill any pathogen that we may ingest. Thirdly, all our "openings" - the trachea, nasal passages, urethra, and vagina, are lined with mucous membrane. The cells of the membrane produce and secrete mucus, the mucus then traps the incoming pathogens. Another point regarding the mucus which is important is that it secretes lysozyme, therefore, it chemically damages the pathogens. Lastly, the cilia serves as defense as well; the cilia is a hair-like extension capable of wave-like movement, the movement of the cilia removes the mucus, excreting the pathogens.
Now I will give a general summary of the immune response process to make everything more clear. The immune response, in itself, begins when a pathogen enters the body. The macrophages that encounter the pathogen ingest, process, and display the antigen fragments on their cell surfaces (antigen-presenting cells). These cells then interact with the T-helper cell to recognize the same antigen. During this "communication" the macrophage releases a chemical alarm (interleukin 1), which stimulates the T-helper cell to secrete interleukin 2, causing proliferation of some cytotoxic B cells and T cells. At this point the process has two directions to go in, one through the B cell and one through the T cell. Since normal cells can also be affected by the pathogens, so the T cells recognizes a particular antigen. Once they are recognized, the T cell binds to the infected cell and releases a chemical to kill the infected cell, resulting in the destruction the pathogen. B cells which are in a wide variety, each recognizes particular antigens. When the B cells are activated by the T-helper cells they are differentiated into plasma cells, which become anti-body producing "centers", sending out antibodies that can bind to the antigen involved in the infection to the blood stream. The antibodies bind to the antigens on the surfaces of the pathogens, marking them for destruction, which is done by macrophages. The remainder of the B cells that don't produce anti-bodies, become memory cells; these memory cells are what trigger an immune response in a future infection by the same pathogen in a faster and more powerful way. This response is what gives our bodies immunity to diseases you have already had or have been vaccinated with.
To end on a different note, I will comment on HIV/AIDS to show how it is an important subject when regarding the immune system. HIV is the human immunodeficiency virus (which leads to AIDS). HIV damages a person's body by destroying specific blood cells - the T cells - which are crucial in helping the body fight diseases. AIDS is caused by HIV, since it is the last stage of HIV infection; it is when a person's immune system is severely damaged and has difficulty fighting diseases and certain cancers. HIV (the virus, not AIDS) can be transmitted from man to woman, woman to man, man to man, or mother to fetus. The man to man, woman to woman, and man to man contamination is during sexual relations with no protection (condoms). It can also be transmitted through breast milk, saliva, or other body fluids. Another physical from it is transmitted in is through contaminated needles and blood transfusions. HIV/AIDS causes many social implications. Firstly, it may cause social stigma and discrimination to the contaminated individuals. Secondly, it may affect their chances of obtaining employment and life insurance. Thirdly, it impacts the costs on health systems to treat contaminated individuals. Furthermore, early deaths reduces number of adults, therefore the workforce, and consequently the family income. Another important problem to the contaminated individuals is that the expense of the drug treatment is very high.
Below are two quizzes that I found on the immune system which is a good way to practice and become more familiar with the topic. The first quiz is on HIV/AIDS and the second quiz (includes an animation) is on the immune system in general.
Below is a video I found regarding a research scientists at Cambridge University did which claims that the immune system's main assault on viruses take place inside infected cells and not outside as it was thought to (came out in 2010).
Below is a brief video I found explaining the basics on immunity.