Immune System Activation

Our immune system has evolved to fight off nearly any pathogen, but it must be activated before it does battle. The illustration summarizes the activation of the two main response modes.

Starting with the cellular immune response, a pathogen in the body is ingested by an antigen-presenting cell (APC), such as a macrophage. It's broken apart inside the APC, and a protein fragment (T-cell epitope) is presented at the cell surface in an MHC-I protein complex. A killer T cell will interrogate the epitope using its CD8 receptor, which binds only to a particular foreign epitope from a particular pathogen. If binding occurs, then small signaling molecules (interleukins) secreted from a mature helper T cell cause the killer T cell to mature. This activated killer T cell then roams the body looking for and killing infected cells.

An APC will also display epitopes in an MHC-II complex, which is examined by the CD4 receptor of a helper T cell (not shown). If binding occurs, the APC will secrete interleukins that make the helper T cell mature.

The mature helper T cell is also needed to activate B cells, which are central to the antibody-based response. The B cell carries a unique receptor on its surface that only binds to specific pieces of a pathogen. Binding causes the B cell to engulf and break the pathogen apart, followed by the display of an epitope in a MHC-II complex on the cell's surface. If a mature helper T cell binds to the epitope, it releases signaling proteins (interleukins) that transform the B cell into an antibody factory. The antibodies will circulate through the body and glom onto their targeted pathogen. This inhibits the pathogen's function and tells a macrophage or other predatory cell to engulf and destroy the intruder.

Both responses require mature helper T cells, the cell that HIV overwhelmingly infects. In targeting and eliminating these cells, the immune system causes its own breakdown, the devastating last stage of which is called AIDS.