Diseases can be caused by inorganic toxins, genetic disorders, or microorganisms. Microorganisms that cause sickness are known as pathogens. All pathogens have antigens on the surface of their cells, such as receptors or carbohydrates, which are features recognized by the human body's immune system.
Introduction to the Immune System
The immune system is an organ system which protects the body from sickness. The immune system's important defenders are white blood cells found in the blood. Two broad classes of white blood cells are lymphocytes and macrophages. Two important types of lymphocytes are B cells and T cells. The macrophages, B cells, and T cells make up the immune response discussed in this tutorial. The immune system has two main defenses: nonspecific defenses and specific defenses.
Nonspecific defenses defend the body in the same way regardless of the type of pathogen. This is an efficient way of filtering out most pathogens from causing the body to get sick with little effort. The physical and chemical barriers, such as skin, hair inside body cavities, mucus, and saliva, can prevent or kill most pathogens before they can enter the body and spread to cause sickness. If tissue is damaged or injured however, the inflammatory response activates. Chemical signals such as histamine are released, signaling macrophages (nonspecific defenders) to come and kill the bacteria trying to spread into the body.
Specific defenses can mount attacks against specific pathogens by recognizing and remembering them. There are two main specific defenses: humoral immunity and cell-mediated immunity. Humoral immunity protects against pathogens in the bodily fluids, whereas cell-mediated immunity protects against cells that have already become infected by the pathogen. Humoral immunity involves B cells and antibodies. There are many diverse B cells which can respond to diverse number of antigens. When a B cell runs into an antigen, it becomes a plasma cell which can release specific antibodies into the blood to kill the antigen, while at the same time, some B cells become memory B cells to remember that specific pathogen. If the body ever gets infected by the same pathogen, the memory B cells awaken and do a massive antibody attack before the pathogen can spread as before. Sometimes, however, cells have become infected with pathogen already, and cannot be killed by humoral immunity. In cell-mediated immunity, T helper cells bind to infected cells to signal Killer T Cells to come and attack the infected cells. The killer T cell can kill the infected cell along with the pathogen by destroying the membrane of the cell.