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 pathogens.
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.