T cells circulate throughout the body until they recognise antigens on the surface of antigen presenting cells. In addition to T cell receptor (TCR) binding to antigen-loaded MHC, require a number of secondary signals to become activated and respond to the threat. In the case the first of these is provided by CD28. This molecule on the T cell binds to one of two molecules CD80 or CD86. This process leads to the production of many millions of T cells that recognise the antigen. In order to control the response, stimulation of CD28 by B7 induces the production of CD152. This molecule competes with CD28 for B7 and so reduces activation signals to the T cell and winds down the immune response.
In dendritic cells, the most important CD markers of activation are CD80, CD86 and CD83. CD40 is already expressed in most immature dendritic cells, although their expression intensity increases strongly, following the activation.
The surface antigens that serves as the B-cell antigen receptor (BCR) has two roles in B-cell activation. First, like the antigen receptor on T cells, it transmits signals directly to the cell's interior. Second, the B-cell antigen receptor delivers the antigen to intracellular sites where it is degraded and returned to the B-cell surface. CD40, a ligand for the B cell, is a key component of activation antigen of T cells.