The immune system is a host defense system comprised of many biological structures and processes within the organism designed to protect against diseases. Even simple microorganisms, such as bacteria, possess a rudimentary immune system.
All immune cells come from “precursor” cells or stem cells, located in the bone marrow. Later on, the immune cells migrate from the bone marrow to other parts of the body (thymus, lymph nodes, spleen), where they finally differentiate into all forms of mature cells that compose the immune system.
In humans, the immune system can be classified into two distinct groups of cells and systems:
Innate Immune System – Also known as the non-specific immune system, its cells recognize and respond to pathogens in a generic way, providing immediate but non-long lasting defense. This system activates the adaptive immune system (described below). Think of the innate immune system as the paramedics of the body; it acts as the first line responders in emergency situations. One of its key cells is known as the Natural Killer (NK) cells, which identify and eliminate those cells that are infected with viruses or transformed by cancer.
Adaptive or Acquired Immune System – Also known as the specific immune system, it is composed of highly specialized cells that, when working properly, are specific to a particular pathogen. This system creates memory cells that provide long-lasting, targeted protection against pathogens or cancer.
The decline of the immune system with age (immunosenescence) is reflected in the increased susceptibility to infectious diseases, poorer response to vaccination, increased prevalence of cancer, accelerated aging (frailty), autoimmune and other chronic diseases. Both innate and adaptive immune responses are affected by the aging process; however, the adaptive response seems to be more affected.