Introdução ao sistema e à resposta imune Um pouco de história Vacinas e outras imunizações

Doença Max. number of cases Number of cases in 2003 Percent change Diphtheria206,939 (1921) Sarampo894,134 (1941) Caxumba152,209 (1968) Coqueluche265,269 (1934)8, Polio (paralytic)21,269 (1952) Rubella57,686 (1969) Tetanus1,560 (1923) Haemophilus influenzae type B ∼ 20,000 (1984) Hepatitis B26,611 (1985)6, Tabela 1-1. Eficiência da vacinação no controle de doenças infecciosas comuns

InnateAdaptive Characteristics SpecificityFor structures shared by groups of related microbes For antigens of microbes and for nonmicrobial antigens DiversityLimited; germline-encodedVery large; receptors are produced by somatic recombination of gene segments MemoryNoneYes Nonreactivity to self Yes Components Physical and chemical barriers Skin, mucosal epithelia; antimicrobial chemicals Lymphocytes in epithelia; antibodies secreted at epithelial surfaces Blood proteinsComplementAntibodies CellsPhagocytes (macrophages, neutrophils), natural killer cells Lymphocytes Tabela 1-2. Características da resposta inata e da adaptativa

FeatureFunctional significance SpecificityEnsures that distinct antigens elicit specific responses DiversityEnables immune system to respond to a large variety of antigens MemoryLeads to enhanced responses to repeated exposures to the same antigens SpecializationGenerates responses that are optimal for defense against different types of microbes Self-limitationAllows immune system to respond to newly encountered antigens Nonreactivity to self Prevents injury to the host during responses to foreign antigens Table 1-3. Cardinal Features of Adaptive Immune Responses

Innate immunityAdaptive immunity PhagocytesNK cellsAntibodiesT & B lymphocytesLymph nodes Invertebrates Protozoa+---- Sponges+---- Annelids++--- Arthropods+---- Vertebrates Elasmobranchs (sharks, skates, rays) +++ (IgM only) +- Teleosts (common fish)+++ (IgM, others?) +- Amphibians+++ (2 or 3 classes) +- Reptiles+++ (3 classes) +- Birds+++ (3 classes) ++ (some species) Mammals+++ (7 or 8 classes) ++ Evolução do sistema imune

Protective immunity against microbes is mediated by the early reactions of innate immunity and the later responses of adaptive immunity. Innate immunity is stimulated by structures shared by groups of microbes. Adaptive immunity is specific for different microbial and nonmicrobial antigens and is increased by repeated exposures to antigen (immunologic memory). Humoral immunity is mediated by B lymphocytes and their secreted products, antibodies, and functions in defense against extracellular microbes. Cell-mediated immunity is mediated by T lymphocytes and their products, such as cytokines, and is important for defense against intracellular microbes. Immunity may be acquired by a response to antigen (active immunity) or conferred by transfer of anti-bodies or cells from an immunized individual (passive immunity). The immune system possesses several properties that are of fundamental importance for its normal functions. These include specificity for different antigens, a diverse repertoire capable of recognizing a wide variety of antigens, memory for antigen exposure, specialized responses to different microbes, self-limitation, and the ability to discriminate between foreign antigens and self antigens. Lymphocytes are the only cells capable of specifically recognizing antigens and are thus the principal cells of adaptive immunity. The two major subpopulations of lymphocytes are B cells and T cells, and they differ in their antigen receptors and functions. Specialized antigen-presenting cells capture microbial antigens and display these antigens for recognition by lymphocytes. The elimination of antigens often requires the participation of various effector cells.

The adaptive immune response is initiated by the recognition of foreign antigens by specific lymphocytes. Lymphocytes respond by proliferating and by differentiating into effector cells, whose function is to eliminate the antigen, and into memory cells, which show enhanced responses on subsequent encounters with the antigen. The activation of lymphocytes requires antigen and additional signals that may be provided by microbes or by innate immune responses to microbes. The effector phase of adaptive immunity requires the participation of various defense mechanisms, including the complement system and phagocytes, that also operate in innate immunity. These mechanisms neutralize and eliminate the microbes and antigens that elicited the response.