Properties of Adaptive Immunity

Occasionally an infectious agent, such as a virus or bacterium, evades the innate immune system and requires that the body mount a specialized immune response specific to the pathogen to control the infection. This immune response must be able to recognize foreign invaders. The immune response must also specifically recognize an infected cell, distinguishing between infected and noninfected cells. An innate immune response is the first line of defense against infection and is nonspecific. An immune response that is specific to a particular pathogen and arises after an immunizing event, such as vaccination or an infection, is known as an adaptive immunity.

Lymphocytes are a subclass of leukocytes, or white blood cells, specific to the adaptive immune response, and the main types of lymphocytes are T cells and B cells. Innate immune cells can mount a nonspecific immune response against a wide variety of different pathogens, while individual adaptive immune cells are only able to recognize a specific pathogen. A B cell, or B lymphocyte, is an adaptive immune cell that reaches maturation in the bone marrow. A T cell, or T lymphocyte, is an adaptive immune cell that is generated in the bone marrow and matures in the thymus. The thymus is an adaptive immune organ located behind the sternum that stimulates T cell maturation with hormones called thymosins. Each of these cell types mounts a different type of immune response. T cells are involved in cell-mediated immunity, by which T cells kill infected target cells through direct cell-cell contact and secretion of cytokines. Each cytokine is one of a family of chemical messengers that regulate several different immune functions. By contrast, B cells do not engage in direct killing and instead initiate humoral immunity, which is mediated entirely by antibodies, or proteins secreted from B cells that circulate throughout the body to initiate killing. Both B and T cells are able to recognize an antigen, a specific portion of a foreign particle that activates an immune response. B cells and T cells both have proteins on their cell surface, called antigen receptors, that recognize a specific antigen that is located on the surface of a cell that has been infected with a pathogen or may directly recognize a pathogen that lives outside of cells—in the bloodstream, for example. B cell antigen receptors are antibodies that are capable of binding two antigen molecules, while T cell antigen receptors are called major histocompatibility complexes that can only bind one antigen molecule. Specificity refers to the ability of an adaptive immune cell to recognize a particular target antigen. When a B or T cell receptor binds to its target antigen, the cell will be activated and become better able to fight the infection through increased cell division and increased secretion of cytokines or antibodies. Immunocompetence refers to the ability of a person to have a normal immune response when challenged by a pathogen. An immune challenge refers to both the introduction of a pathogen and the immune response initiated by the introduction. One way that adaptive immune cells are able to fight an infection is by rapidly proliferating, or growing quickly in number. Memory B and T cells remain in the system so that the immune system can react faster upon the second exposure to the antigen. Each pathogen will receive its own set of B and T cells, and over the course of a lifetime, hundreds of different antigens will be recognized. Proliferation of B and T cells that recognize an antigen increases the number of cells capable of fighting a particular infection. A subset of the human population is immunocompromised due to advanced age, medical treatments, infection with certain viruses, or poor nutrition. Immunocompromised refers to a condition of the immune system that is suppressed or inactive. These conditions are most often mediated through leukocytes, including B and T cells.

An adaptive immune response, and activation of B and T cells, can occur in one other way besides being infected. Vaccination can also lead to an adaptive immune response. A vaccine is a nonpathogenic substance that is used to generate a protective immune response against a particular disease. Childhood vaccines, such as those against chicken pox or the measles, confer lifelong resistance to the infection by the inoculated pathogen. One benefit of an adaptive immune response is that in addition to curing an ongoing infection, it provides protective immunity that can last for years following the infection or vaccination event.

An adaptive immune response involves four successive steps. As an immune response progresses, the immune cells involved in fighting an infection become more specific to the pathogen before eventually transitioning to a long-lasting and specific memory phenotype. The four phases of an adaptive immune response are the activation, proliferation, effector, and memory phases. The progression through these phases creates progressively more specific adaptive immune cells in the body.

Lymphocytes of the adaptive immune system are initially naive. Naive B cells and T cells are mature, but have not yet been exposed to an antigen and can not generate an immune response. Activation of B and T cells occurs when they encounter antigen-presenting cells that are presenting the specific antigen that the B or T cell can recognize. These activated cells are known as effector cells. Once activated, B and T cells can rapidly proliferate. This creates a population of effector B and T cells. Each B cell in a population of effector B cells specific to a particular antigen is a B cell clone. Each T cell in a population of effector T cells specific to a particular antigen is a T cell clone. Next, during the effector phase, these B and T cells work to clear the infection. B cells do this by secreting antibodies, proteins that are similar in structure to antigen receptors but are secreted from cells instead of being bound to the cell surface. Antibodies will bind to infected cells or pathogens, and this directs immune cells and proteins called complement to kill the pathogen or infected cell. T cells clear infections by moving throughout the body, seeking out infected cells, and killing them.

After the immune response has persisted for several days or weeks, the memory phase begins. Immune memory is the ability to deliver long-lasting, protective immunity against a pathogen. If a person encounters the same viral, fungal, or bacterial infection again in the future, the immune response can progress extremely rapidly by reactivating these specific memory cells instead of beginning the process with naive cells. A secondary response is an immune response that occurs any time a pathogen is encountered subsequent to an initial infection that prompted the development of memory cells against the pathogen’s antigen.