The Immune System
The immune system is a collection of specialized cells, plasma proteins, and molecules that serve to protect us from harmful substances. The system is composed of two complementary components: the innate and the adaptive (or acquired) immune response. Each system is especially designed to recognize and eliminate any foreign agent (antigen) that invades our bodies.
The Innate Response
The innate system is a quick response mechanism composed of the epithelia of the (epidermis, gastrointestinal, respiratory, genital-urinary, reproductive tracts), several types of phagocytic cells (macrophages, Natural Killer, and neutrophils), dendrites, and various plasma proteins (members of the complement system). Epithelial cells and epithelial mucosa provide both physical and chemical protection against microbes. The tightly bound epithelia block the entry of microbes, while the mucosa secretes antimicrobial proteins and lymphocytes that effectively kill infectious agents. Microbes that breach these initial defenses are met by cells that detect their presence and recruit phagocytes to kill them. The resulting inflammation (heat, redness, swelling and pain) results from the migration of these various cells to the area of infection. The inflammation occurs as the many cells converge on a microbe; the pus associated with infections is released by neutrophils (a type of macrophage) as they die.
The Adaptive Response
Antigens (microbes or any foreign substance) that evade the innate response trigger the adaptive immune system. Dendrites will initially take the substance into the lymphatic system thence to the lymph nodes where T (thymus derived) and B (bone marrow derived) lymphocytes activate the immune response. These cells form two distinct components of the adaptive system: the humoral response, which employs B lymphocytes to respond to extracellular toxins and microbes, and the cell mediated response which uses T cells to defend against intracellular toxins. A second important distinction is that T cells are active phagocytes that travel to and eliminate antigens. B cells secrete antibodies (also called immunoglobulin) that respond to antigens in a variety of ways: IgG and IgM identify microbes and marks them for phagocytosis, IgA eliminates pathogens from epithelial mucosa, IgE protects against parasites: IgG is also active in the placental tissue where it protects newborns until their immune systems develops.
Long-term immunity results when B cells “remember” the characteristics of a given antigen. When injected with a vaccine (a weakened strain of a given pathogen) the adaptive system activates B-lymphocytes that secrete antibodies that recognize and target the antibody. The B cells also form memory B cells that will remember the specific characteristics of the antigen. These B cells will provide protection in the event of a real infection.
Evasion of Immune Responses
Our immune system is well equipped to provide protection from most pathogens. Our genome is well suited as a result of the evolutionary pressures that have selected for protections against such a wide array of pathogens. Pathogens, however, continue to evolve and develop new strategies and mechanisms to evade immunological responses. Many pathogens are able to alter their surface proteins and thus avoid detection by our immune system. This antigenic variation allows viruses to infect host cells before they are detected.
Our adaptive immune system has developed an array of mechanisms as a defense against antigens. In order to attack antigens, the immune system must first recognize the intruder. Many microbes have developed mechanisms that allow them to change their surface proteins so that they become unrecognizable to the immune system. Many of these mechanisms occur at the genetic level allowing pathogens to modify their surface antigens or cellular processes so that they can disable, confuse, or disrupt immune responses. Well-known examples are the HIV and influenza viruses that are able to regularly recombine their RNA sequences so that they express a different genetic makeup. This is the reason that a vaccine for HIV has been so difficult to synthesize and why we need new strains of influenza vaccine each season.
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