Infections that are not destroyed by this attack attract the attention of lymphocytes. Then they engulf and digest what they have damaged, a process called phagocytosis. They secrete highly destructive substances including enzymes that digest proteins and reactive chemicals such as bleach that kill. Both of these cell types are effective killers in their own right. Early in an immune response, the most important of these are the innate immune system cells neutrophils and macrophages, which are the first at the scene of an environmental breach, such as an insect bite. Tackling infections is the job of different types of white blood cell. If cells detect such telltale signs they respond, by producing cytokines that serve as alarm signals for surrounding tissues, and by committing rapid and effective suicide that leaves a cell remnant that can initiate adaptive immunity directed at the inciting infection. Cells use a sophisticated system for sampling the proteins they are making, to check that none have come from viruses. Receptors within the cytoplasm can bind to virus-derived signature molecules, such as different types of nucleic acid and signal that infection is present. Organisms such as viruses can spend most of their life hidden in the complicated cytoplasm of the cell, making them difficult to recognize from the outside. Receptors are also present inside the cell where they play an important role, acting to detect evidence of infection. In the course of an immune response, cells follow a programme, such that the overall outcome maximizes the likelihood of surviving and eliminating infection or cancer. The receptors associated with the immune system are generally concerned with interrogating the environment for evidence of danger, infection or abnormal cell death. ![]() Many carry out fundamental functions, such as transporting glucose into the cell. Cells express hundreds of different types of receptor on their surface. Receptors bind ligands which may be receptors on other cells, or soluble molecules such as cytokines. Among these proteins are receptors by which cells interact with each other and the environment. ![]() An immunologist or a haematologist may subdivide these populations further, on the basis of the proteins that are expressed in their cell membranes. In a routine blood test, five different kinds of white blood cell will be counted ( Table 1). Many important immune cell types have been identified. Some of these then undergo very significant secondary education before they are released to patrol the body. After early childhood, most immune cells are produced from the bone marrow. For this reason, although the immune system may seem a less substantial thing than an organ such as the heart or the liver, in aggregate, immunity consumes enormous resources, producing the large number of cells that it depends on for successful functioning. It is a fundamental property of immunity that no part of our body is cut off from its surveillance. Managing this threat depends on understanding how to maximize the potential of our sophisticated immune system in the service of human health. Emerging infectious diseases have as much potential to shape future human history as the epidemics and pandemics of the past. The pressure that natural selection exerts is inexhaustible and unending. ![]() The problems that the mammalian immune system solves are not restricted to higher animals they are faced by all forms of life and are ignored by none.
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