As you have learned, CD4 cells are involved in the "organization" of the specific immune response. Their function is to tell other immune system cells about a specific invasion. Let's first explore how CD4 cells communicate in the Antibody-Mediated Response.

The Antibody-Mediated Response involves B lymphocytes and Antibodies. B Lymphocytes are derived from the same progenitor lymphoid cell as the T cells but has undergone a different differentiation pathway. The structure of a B lymphocyte is somewhat similar to a T cell in that it has a structure on its surface that recognizes a specific antigen. However, the structure on the B cell is not called a "receptor"; it is an ANTIBODY. Antibodies are proteins . All antibodies have a Y-shaped as shown below.

At the tips of the Y are two identical structures that serve as the Antigen-binding site. The structure of the Antigen-binding site differs between B cells, just as the structure of the T cell receptor differs between T cells. Your body has the capacity to make antibodies to any possible pathogen that invades it ... that is a lot of antibodies! And this number of proteins would account for more than the 20,000 genes the human body has. So how possibly can your body make so many DIFFERENT antibodies if it doesn't have that many different genes?

Hopefully you thought of alternative splicing! The diagram above illustrates how from one set of genes our cells can produce so many different antibodies.

So just as we saw with the lonely Helper T cell roaming the body looking for the pathogen it was designed to fight you also have lonely B cells doing the same thing. However B cells act like macrophages when they find their pathogen and become APCs. Once a B cell has located its pathogen, engulfed it, and become an APC it is now considered activated and ready to be told what to do by a Helper T cell. This animation (Links to an external site.) and the diagram below shows the interaction between a CD4 cell that is specific for the same pathogen as an activated Antigen-presenting B cell. k

The CD4 cell and the activated antigen-presenting B cell interact via the antigen being presented and the T cell receptor. This is a similar interaction as we saw between the Helper T cell and the macrophage. It is also important to note that the B cell presents the antigen with an MHC II molecule. The interaction between these two cells initiates a cell signaling pathway by the CD4 cell. The pathway results with the production and secretion of interleukin that acts a signal to the B cell. This signal indicates to the B cell to undergo mitotic cell division resulting in an army of B cells that produce the same structural antibodies as the original. The diagram below illustrates this process.

As you can see in the diagram, this army of B cells actually undergoes more differentiation where most of the cells become plasma cells. Plasma cells are B cells that produce antibodies that are secreted into the body. These cells are filled with Rough Endoplasmic reticulum and are serious protein factories producing thousands of antibody molecules. The antibodies released into the body then go and seek out pathogens that bear an antigen that fits its antibody. When an antibody binds to the pathogen it essentially "handcuffs" it and holds it for a macrophage, or other digesting white blood cell, to come and eat up the pathogen.

This animation illustrates the complete Antibody-Mediated Response. Please Watch!!!

I want to call your attention to two other details about this process.

The antibody-mediated response is effective at locating and eliminating pathogens that are free-floating. In other words, the antibodies could only capture a pathogen that was not within a cell. If a pathogen was inside of a cell it would essentially be hidden from a B cell response.
A second differentiation also occurred with the B cells - Memory B cells were also produced. These cells play a role in long-term immunity and we will return to these after the Case Study.