|MadSci Network: Immunology|
Your question is a very good one. I'll try to answer in plain English. Basically both the humoral and cell-mediated immune responses are directed at protecting the body from infection. (There are some other things the immune system can sometimes do also, like fight cancers, however for simplicity we'll focus on fighting infections.) Different types of pathogens which enter the body require different types of responses to effectively clear the body of the pathogen and prevent some of the dangerous effects of the pathogen. This is why the immune system can generate these two types of responses. One should note, that often both types of responses are going on in concert, and each can be having a beneficial effect.
Lets first take the example of a virus infection. Viruses are incabable of replicating on their own, and so they must invade cells of our body and hijack the cell's machinery to replicate. This means that the majority of virus present in our body during an infection is usually contained within our own cells. Cell mediated immunity is the type of response that can allow cells of our immune system (T cells) to recognize which cells in our bodies are virally infected. The T cells seek out these cells, and destroy them, thereby killing the virus contained within them. Once all of the infected cells are wiped out, there is no more virus left in our bodies. By generating a response to the virus, our T cells end up with what is called memory. Next time we are infected by the same virus, certain T cells remember seeing it before and can respond quicker and more explosively. This is the basis behind vaccination for certain diseases. One thing to note is that some viruses place their genetic material into the genetic material of the cell and hide out. T cells may not recognize that this cell is infected, and may pass over it when seeking out infected cells. At a later time point, using the genetic material which the virus deposited in the cell at an earlier time, the cell may begin producing new virus. Now the cell should be recognized by T cells, and destroyed.
Now lets talk about the humoral immune response. This refers to a response by certain cells of the immune system (B cells) which have the ability to make what are called antibodies. Antibodies are proteins which float around in the blood and have a particular ability to bind to a foreign protein. Depending on the features of this foreign protein, antibody binding to it will have numerous possible effects. One example would be antibodies which can recognize proteins on the surface of invading bacteria. If antibody coats the surface of the bacteria, the cell will more easily recognize that the bacteria is foreign, and try to destroy it. Another example is antibody which can recognize a certain toxin produced by bacteria and neutralize its toxic action. An example of this is antibody against tetanus toxin. If we get infected with the bacteria which causes tetanus, we can usually handle the bacteria infection and rid the body of the pathogen. The problem is that the bacteria can quickly make a potent toxin which can make us very sick. We need antibodies to bind the toxin and prevent it from acting. This is why we get vaccinated against tetanus. The vaccine is actually an inactive form of the toxin. By putting this foreign protein in the body, our B cell recognize it as foreign and produce antibody against it. This antibody floats around our bodies for years, until the day we get infected by the tetanus bacteria. As soon as the bacteria makes the toxin, the antibodies bind it, and prevent any terrible consequences. This is an example of a humoral immune response.
Let me note that antibodies can also be beneficial against some virus infections, if they can bind to virus particles and prevent them from getting in to our cells. The humoral immune response would be a first line of defense against the virus. Any virus that gets by the antibody and manages to infect our cells would now have to be dealt with by the cell mediated immune response (the recognition and destruction of the infected cells). This is an example of both systems working together.
I hope this response was helpful, and not too rambling.
Try the links in the MadSci Library for more information on Immunology.