Adaptive immunity is defined by the invocation of a specific cellular immune response, in contrast to innate immunity that is distinguished by a non-specific immunity. Adaptive responses involve lymphocytes and are characterized two ways, humoral and cell-mediated immunity. B cells mediate a humoral response, the creation of antibodies to bind specifically to foreign antigens including bacteria, and microbial toxins. Cell-mediated immunity is a more complex system involving the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokine release in response to an antigen and is mediated by T-lymphocytes. T cells recognize a pathogen only after antigens (small fragments of the pathogen) have been processed and presented typically in combination with a “self” receptor called a major histocompatibility complex (MHC) molecule. Cell-mediated immunity is important in combating intracellular organisms, performing tumor surveillance, mediating transplant rejection, and fighting fungal and viral infections.