The immune system recognizes invaders in a complex way. The two lymphocyte groups use different strategies. B cells can attack the intruder directly. T cells require assistance from B cells or other white blood cells that ingest and digest foreign invaders. Protein fragments from the processed invader reappear on the surface of these cells, bound in specialized grooves of a complex of proteins. This complex, known as MHC (major histocompatibility complex) proteins, presents the invader fragments to T cells. The T cell receptors recognize the bound protein complex and initiate a cascade of events, enlisting the B cell army as well as other T cells. This system allows lymphocytes to identify and destroy cells in which viruses or bacteria are hidden and multiplying.