What is the difference between humoral and cell-mediated immunity? What role do lymphocytes play in immunity?

Cell mediated immunity involves a certain lymphocyte(white blood cell) known as the T cell.

Humoral Immunity involves a certain lymphocyte known as the B cell.

These are simply two different strategies in which your body fights invaders-foreign particles known as antigens.

A cell known as the Macrophage is the security patrol in your blood.

##color(green)((MACROPHAGE)## When it detects an invading particle( antigen ), it eats it up and minces it into tiny fragments. It then takes one of those fragments and presents it to the T Helper Cell(a type of lymphocyte) who is basically the General of the bodies' army of white blood cells.

From here, the General(T Helper Cell) can take two courses of action depending on how serious the situation is. He has to defeat a whole army of antigens, so he must act prudently. ##color(white)(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx## ##color(purple)(CMI##

If the antigens have invaded cells of the body then drastic measures have to be taken. The CMI Strategy is deployed. CMI = Cell Mediated Immunity.

T Helper Cell(the General) initiates production of Cytotoxic T Cells(also called T Killer cells-a fitting name btw) by sending out chemical signals. These Cytoxic T cells search out the infected cells, bind to them, and release a barrage of chemical ammunition that destroys the cell along with the antigen in it. So your cell just got destroyed by your own body. Desperate times call for desperate measures. Without it the infection could have spread to other cells until it was everywhere.

(Overview CMI Strategy) ##color(white)(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx##

##color(purple)(HR##

However, if the antigen has not yet infected any cells, then the General (T Helper cell) can deploy the HR strategy. HR = Humoral Response.

Remember how the security patrol ate one of the antigen and diced it up? From that, the General gets an idea. He gives the patrol the ability to eat and dice up all the antigens lurking freely in the bloodstream.

To do this, the General(T helper cell) sends out chemical signals to B cells( a type of Lymphocyte). There are a lot of them in reserve. So only a few are needed.

Those few get converted to plasma cells who then get to work on producing antibodies. This conversion of some B cells is called clonal selection . Antibodies are chemicals that bind to the surface of the antigens floating around in the blood.

(In Action: Antibodies going after an antigen in the bloodstream)

The antibodies make it easy for the patrol (macrophages) to find the antigens. The macrophages then engulf the antigen along with the attached antibody, slicing both to pieces, while at the same time neutralizing the threat. The selfless antibodies sacrifice their lives so that you can live yours.

(Overview HR Strategy)

To learn more about how this amazing body of yours keeps you healthy, you can watch this nice animation which shows you how this immunity process actually happens in real life:http://highered.mheducation.com/sites/0072943696/student_view0/chapter14/animation__the_immune_response.html