The immune system is composed of the bone marrow, lymph nodes, lymphatic vessels, the reticuloendothelial system, spleen, and thymus. When a foreign substance (antigen) is recognized, the immune system responds rapidly in a highly specific and well-coordinated manner. The resulting immunological event leads to an inflammatory response, killing of the invading microbial agents, and disposal of the foreign toxic compounds. Primary immunity (also called natural immunity) is the first line of defense against pathogenic invaders and does not require sensitization or prior exposure to pathogens (antigens). It includes: mucocutaneous barriers (e.g. lining of the mouth, throat and lungs), and phagocytosis by white blood cells (engulfmant of foreign matter). Secondary immunity (also called acquired immunity) refers to immune function that is activated after prior exposure to a pathogen. When a minor injury occurs and the skin is penetrated, this can introduce a foreign agent into the body. This elicits an immune response in which basophil cells release histamine, which causes capillary dilation allowing fluid to enter the surrounding tissue, causing inflammation of the local area. Macrophages and neutrophils squeeze out of the capillary into tissue and phagocytosize (devour) microbes (bacteria and viruses) as well as old blood cells, bits of dead tissue, and other debris. Pus consists of the remains of dead tissue, cells, bacteria, and living white blood cells. In addition, plasma proteins called complement can be activated by injury and foreign invasion. These proteins form pores in bacterial cell walls and membranes which allow fluid and salts to enter the bacterium. This causes the bacterial cell to expand until it bursts. Complement also releases chemicals which attract macrophages to the site of infection and induce inflammation (i.e. "complements" other immune responses). An antigen is a material or protein which the body does not recognize as self (microbe, foreign cell, cancer cell). The ability of the body to distinguish self from non-self involves two types of lymphocyte cells: B cells (matures in the bone marrow) and T cells (matures in the thymus). return to lecture