B7 (protein)

B7 is a type of peripheral membrane protein found on activated antigen presenting cells (APC) that, when paired with either a CD28 or CD152 (CTLA-4) surface protein on a T cell, can produce a costimulatory signal or a coinhibitory signal to enhance or decrease the activity of a -TCR signal between the APC and the T cell, respectively. Binding of the B7 of APC to CTLA-4 of T-cells causes inhibition of the activity of T-cells.

There are two major types of B7 proteins: B7-1 or CD80, and B7-2 or CD86. However, it is not known if they differ significantly from each other. CD28 and CTLA-4 each interact with both B7-1 and B7-2.

There are several steps to activation of the immune system against a pathogen. The T cell receptor must first interact with the (MHC) surface protein. The CD4 or CD8 proteins on the T-cell surface form a complex with the CD3 protein, which can then recognize the MHC. This is also called "Signal 1" and its main purpose is to guarantee antigen specificity of the T cell activation.

However, MHC binding itself is insufficient for producing a T cell response. In fact, lack of further stimulatory signals sends the T cell into anergy. The costimulatory signal necessary to continue the immune response can come from B7-CD28 and CD40-CD40L interactions. There are other activation signals which play a role in immune responses. In the TNF family of molecules, the protein 4-1BB (CD137) on the T cell may bind to 4-1BB ligand (4-1BBL) on the APC.

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