Every organism is exposed to substances that can cause harm to its body. Organisms protect themselves in many ways, including physical barriers and chemical neutralizers. Vertebrate animals have an advanced protective immune system. The immune system consists of a complex network of organs that contain cells used to recognize and destroy pathogens (disease-causing organisms or viruses) in the body. Pathogens are typically infectious microorganisms such as bacteria, fungi, and other parasites. The immune system also recognizes viral pathogens, parasites, and cancer cells.
There are many types of cells in the immune system. All immune cells are derived from stem cells in the bone marrow, but the differentiation of the cell types follows different pathways. Macrophages are cells of the innate immune response that engulf and destroy pathogens. Other cells of the innate immune system release chemicals that cause inflammatory responses that inhibit the growth and reproduction of pathogens, recruit other immune cells to the site of infection, and help body cells defend themselves against infection.Lymphocytes are white blood cells that play an important role in the adaptive immune response. These white blood cells are highly differentiated, which means stem cells undergo many rounds of differentiation or processing in order to develop into a lymphocyte. There are two different types of lymphocytes: B cells and T cells. B cells initiate a specific immune response against extracellular pathogens. They produce antibodies, blood proteins that bind to pathogens. Bound antibodies can initiate a process that kills the pathogen directly or can signal cells of the innate immune system to engulf and destroy the pathogen. T cells are processed in the thymus. They can differentiate into different cells, such as cytotoxic T cells or helper T cells. Helper T cells produce chemicals that aid B cells, cytotoxic T cells, and cells of the innate immune system. Cytotoxic T cells recognize and destroy cells that contain intracellular pathogens. There are two types of immunity: adaptive and innate. Innate immunity is a natural immune defense that all animals possess. This includes barrier defenses, including physical, chemical, and mechanical defenses. Barrier defenses include the skin, which functions to keep foreign objects out of the interior of the body. The mucus found in body openings constitutes a chemical defense, and the movement of foreign objects out of the body through ciliary action or coughing and sneezing are mechanical defenses.
Defensive cells of the innate immune system are activated if a pathogen passes the physical and chemical barriers. A very broad range of pathogens is recognized in innate immunity, and the response is non-specific. The macrophage is an example of a nonspecific immune cell. This cell type releases chemicals called cytokines that induce an inflammatory reaction in the body to inhibit the growth of pathogens and prevent the infection of tissue. These cells ingest and destroy foreign organisms in a process called phagocytosis. Cells of the innate immune system also release chemicals that recruit cells of the adaptive immune system to the site of infection.
Adaptive immunity is activated after the innate immune response begins, and it develops more slowly. The adaptive immune system mounts a response that is specific to the pathogen via specialized cells that recognize and destroy the pathogen. Lymphocytes of the B and T cell lineages are the primary cells of the adaptive immune system. B cells have receptors on their surface that recognize molecular structures on the surface of pathogens. Once a B cell receptor binds its ligand, the B cell releases antibodies that bind to specific molecules on the surface of the pathogen. These molecules are recognized by macrophages and increase the phagocytosis of the pathogen. The antibodies also trigger a chemical process that destroys the pathogen, called the complement cascade. B cells are effective against extracellular pathogens, such as most bacteria. T cells also have a receptor that recognizes specific molecular structures; however, these structures are presented to the T cells by cells that have been infected. T cells with CD4 accessory molecules are called helper T cells and release cytokines that help both B and T cells function. T cells with CD8 accessory molecules are called cytotoxic, or killer, T cells. Cytotoxic T cells release chemicals that destroy infected cells. T cells are effective against intracellular pathogens such as viruses. The adaptive immune response is stronger and faster upon repeated exposure to the same pathogen. This is due to the generation of memory cells during an immune response. These cells are already primed to react, should the same pathogen be encountered again.