The immune system consists of various organs and a network of biological processes in the body that detects pathogens and protects an organism against disease. It also provides resistance to infection and toxins. Damage to the immune system can cause autoimmune diseases, inflammatory diseases, and cancer. Immunodeficiency occurs when the immune system is poorly nourished and less active than normal, resulting in life-threatening infections. There are two major subsystems of the immune system: the innate and adaptive.
The innate immune system provides an immediate preconfigured response to broad groups of situations and stimuli. Cells in the innate immune system use pattern recognition receptors to recognize structures that are produced by pathogens. Some leukocytes (white blood cells) act like independent single-celled organisms and are the second arm of the innate immune system.
Cytokines are peptides (chains of small proteins) secreted by other types of cells of the immune system for signaling/communication, similar to hormones. They include chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors. They modulate the balance between humoral (body fluids) and cell based immune responses. They regulate the maturation, growth, and responsiveness of particular cell populations. They also enhance or inhibit the action of other cytokines in complex ways.
Leukocytes include phagocytes
(macrophages, neutrophils, dendritic cells), which identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing and then killing microorganisms.
Phagocytes include neutrophils and macrophages that patrol the body in pursuit of invading pathogens, but can be called to specific locations by cytokines. Neutrophils are the most abundant type of phagocytes which are normally found in the blood stream. They represent 50%-60% of total circulating leukocytes. During acute inflammation, neutrophils are the first cells to migrate towards the site of inflammation (infection) in a process called chemotaxis.
Macrophages are versatile cells that reside within tissue and produce an array of chemicals including enzymes, complement proteins, and cytokines. They also act as scavengers that rid the body of worn-out cells and other debris and as antigen-presenting cells (APC) that activate the adaptive immune system.