Basically, the immune system consists of leukocytes (white blood cells), cytokines, spleen, thymus, bone marrow, intestinal flora, and skin. There are several types of white blood cells, such as basophils, monocytes, neutrophils, eosinophils, mast cells, and lymphocytes; they are found in the blood plasma and their function is to identify, kill, and phagocytose (engulf) foreign microorganisms such as bacteria, viruses, fungi, protozoans (parasites), and toxic proteins, inducing the reproduction of antibodies and lymphocytes T cells. Cytokines are types of proteins which trigger a chemical alarm, signalling the presence of pathogens for the white blood cells to act.
The immune system have two organs: the thymus and the spleen, which are the places where most leukocytes mature. Most leukocytes originate from hematopoietic stem cells in the bone marrow and then go in the blood stream to the thymus and spleen where they mature get stored. The intestinal flora is a natural barrier composed of millions of harmless and useful bacteria, thriving in the first section of the large intestine; these bacteria kill and engulf foreign harmful bacteria. The skin is the first line of defense, insulating the organism from the outside world.