Immune system acts as defense mechanism
Inside our body, there is a mechanism designed to defend us from millions of bacteria, microbes, viruses, toxins and parasites.
An immune system is a collection of mechanisms within an organism that protects
against disease by identifying and killing pathogens and tumor cells. It detects
a wide variety of agents, from viruses to parasitic worms, and needs to distinguish
them from the organism′s own healthy cells and tissues in order to function properly.
Detection is complicated as pathogens adapt and evolve new ways to successfully
infect the host organism. Several mechanisms evolved to recognize and neutralize
The first, called innate immunity, is present before any exposure
to pathogens and is effective from the time of birth. Innate defenses are largely
nonspecific, quickly recognizing and responding to a broad range of microbes regardless
of their precise identity.
The second major kind of defense is acquired immunity, also called
adaptive immunity. Acquired defenses are highly specific that is, they can distinguish
one inducing agent from another, even those that differ only slightly. This recognition
is achieved by white blood cells called lymphocytes, which produce two general types
of immune responses. In the humoral response, cells derived from B lymphocytes secrete
defensive proteins called antibodies that bind to microbes and mark them for elimination.
In the cell–mediated response, cytotoxic lymphocytes directly destroy infected
body cells, cancer cells or foreign tissue. Cellular and chemical components of
these two kinds of defense together protect vertebrates from various threats.