Answer
CD4 cells and CD8 cells are two large categories of naive T-cells divided on the basis of the two cell differentiation surface glycoproteins , CD4 and CD8 proteins-- distinct from T-cell antigen receptors CD4 proteins are found on the surface of cells that differentiate into T-helper and T-regulatory cells. CD8 proteins are found on the surfaces of cells that become T-cytotoxic cells.
A CD4 cell is not a T-cell until it is activated, and an activated T-cell must go through a two-step process to be effective in cell-mediated immune responses. The T-cell activation comprises antigen presentation and costimulation.
Antigen presentation: Naive T-cells cannot recognize antigens. to make them effector T-cells, antigens must be presented to them. This is the role of antigen presenting cells (APCs). Some kinds of cells that function as APCs in this process are dendritic cells, macrophages, and B cells. The antigens must be presented on Major Histocompatibility proteins Type one or Type II. For a CD4+T= cell to recognize an antigen it must be presented by a MHC II protein at the surface of an APC.
MHC Type II proteins are synthesized by ER of APC cells. When an APC comes into contact with a foreign entity/substance (antigen), it engulfs the antigen ; the antigen is then is processed( broken down) by proteases within a phagolysosome, and binds antigen fragments in groves of the MHC II protein. The MHC-antigen complex is exported in vesicles to the surface of the APC cell and displayed for the recognition of CD4 cells ( naive T cells).
With several antigenic signals displayed on its surface, an APC cell signals CD4 cells to respond--mount an immune response. The CD4 proteins are adhesion molecules that bind t CD4+T-cells to the MHC II+antigen complex. But there are other surface proteins on the CD4 cells. These are the T-cell receptors (TCR proteins). These receptors perform the double function of recognizing MHC II proteins as self, and identifying the antigen fragment as non-self. Binding of the CDR-cell TCR receptor to the antigen triggers chemical reactions that effect T-cell activation. Activated CD4 cells mature and differentiate into
helper T- cells (T-c and T- regulatory cells(T-r).
Work Step by Step
Costimulation: This is a process that is necessary to enable immune activity by activated CD4+T-cells.
APC cells in tissues that have been damaged or invaded by antigens ( cells or toxins) secrete proteins called costimulatory proteins-- for, example B7 and CD28. The binding of a costimulatory protein to its receptor on the a T-cell is essential for T-cell effectiveness. If the T-cell binds to the antigen before costimulation takes place, it becomes tolerant of the antigen (anergy is the result). However, if cotimulation takes place , the binding of the antigen complex and the T-cell results in growth of the T-cell followed by formation of a T-cell clone. Some members of the clone will become effector cells and mount the necessary primary response; others will become memory T-cells ready to respond quickly on subsequent challenges.
