ABSTRACT
Immune response is the main means by which an individual is protected from the harmful effects of infectious organisms (such as bacteria, viruses, fungi, and parasites) and other noxious insults. It is a complex system, involving many cell types dispersed across many locations in the body and moving between those locations in the lymph and the bloodstream. In some locations, immune cells are organized into distinct organs, termed primary lymphoid organs, where immune cells arise and mature (bone marrow and thymus), and secondary lymphoid organs (lymph nodes, spleen, gut-associated lymphoid tissue), where mature immune cells interact and respond to antigens. Functionally, the immune system has two divisions, called the innate (also called the natural) and the acquired (also called the specic or adaptive) immune systems.
