Chapter 13Describe how the third line of defense is different from the other two.- When host barriers and nonspecific defenses fail to control an infectious agent, a person with a normally functioning immune system has a mechanism to resist the pathogen-the third, specific line of immunity.- This sort of immunity is not innate but adaptive; it is acquired only after an immunizing event such as an infection.- Acquired specific immunity is the product of a dual system that we have previously mentioned –the B and T lymphocyte.o During development, these lymphocytes undergo a selective process that prepares them for reacting only to one specific antigen or immunogen.o During this time immunocompetence, the ability of the body to react with countless foreign substances develops.- Antigens or immunogens are defined as molecules that stimulate a response by T and B cells.o They are usually protein or polysaccharide molecules on or inside all cells and viruses, including our own.o In fact, any exposed or released protein or polysaccharide is potentially an antigen, eventhose on our own cells.o Our own antigens do not usually evoke a response from our own immune systems. So it is acceptable to think of antigens as foreign molecules that stimulate an immune response.- In chapter 12, we discussed pathogen-associated molecular patterns (PAMPs) that stimulate responses by phagocytic cells during an innate defense response.o While PAMPs are molecules shared by many types of microbes that stimulate a nonspecific response antigens are highly individual and stimulate specific immunity.- Two features that most characterize this third line of defense are specificity and memory.- Unlike mechanisms such as anatomical barriers or phagocytosis, acquired immunity is highly selective.o For example, the antibodies produced during an infection against the chickenpox virus will function against that virus and not against the measles virus.- The property of memory refers to the rapid mobilization of lymphocytes that have been programmed to ‘recall’ their first engagement with the invader and rush to the attack one again.List the four stages of a specific immune response.- The elegance and complexity of immune function are largely due to lymphocytes working closelytogether with macrophages.- To simplify and clarify the network of immunologic development and interaction, we present it as a series of stages. The principal stages are as follows:o Development of B- and T-lymphocyte specificity and migration to lymphoid organs.o Antigen processing by dendritic cell and presentation to lymphocytes; assistance to B cells by T cells.o Lymphocyte activation, clonal expansion, and formation of memory B and T cells.o End result of lymphocyte activation: Antibody release Cell-mediated immunity.Discuss the role of cell markers in the immune response.- All cells – both foreign cells and ‘self’ cells – have a variety of different markers on their surfaces,each types playing a distinct and significant role in:o Detectiono Recognitiono Cell communication- Cell markers play important roles in the immune response, serving to activate different components of immunity.Describe the major histocompatibility complex in two sentences.- One set of genes that code for human cell markers or receptors is the major histocompatibility complex (MHC).- This gene complex gives rise to a series of glycoproteins (called MHC molecules) found on all cells except red blood cells.- The MHC is also known as the human leukocyte antigen (HLA) system.- This marker set plays a vital role in recognition of self by the immune system and in rejection of foreign tissue.- Three classes of MHC genes have been identified:o Class I genes code for markers that appear on all nucleated cells. They display unique characteristics of self and allow for the recognition of self molecules and the regulation of immune reactions. The system is rather complicated in its details, but in general, eachhuman being inherits a particular combination of class I MHC genes in a relatively predictable fashion.o Class II MHC genes also code for immune regulatory markers. These markers are found on macrophages, dendritic cells, and B cells, and are involved in presenting antigens to Tcells during cooperative immune reactions.o Class III MHC genes encode proteins involved with the complement system, among others.Summarize the maturation process of both B cells and T cells.- The maturation of T cells and the development of their specific receptors are directed by the thymus gland and its hormones.o In addition to the antigen-specific T-cell receptor, all mature T lymphocytes express co-receptors called CD3. CD3 molecules surround the T-cell receptor and assist in binding.o T cells also express either a CD4 or a CD8 co-receptor. CD4 is an accessory receptor protein on T helper cells that binds to MHC class II molecules. CD8 is found on cytotoxic T cells, and it binds to MHC class I molecules.o Like B cells, T cells also constantly circulate between the lymphatic and general circulatory system, migrating to specific T-cell areas of the lymph nodes and spleen. It has been estimated that more than 109 T cells pass between the lymphatic and general circulations per day.- B cells were not known in humans for a long time.o Now it is known to be certain bone marrow sites that harbor stromal cells. These huge cells nurture the lymphocyte stem cells and provide chemical signalsthat initiate B-cell development.o As a result of gene modification and selection, hundreds of millions of distincy B cells develop. These naïve lymphocytes circulate through the blood, ‘homing’ to specific sites in the lymph nodes, spleen, and gut-associated lymphoid tissue (GALT), where they adhere to specific binding molecules. Here they will come into contact with antigens throughout life.o There are less B cells in circulation that T cells.- Other information on T and B cells: o When challenged by antigen, both B cells and T cells proliferate and differentiate.o The multiplication of a particular lymphocyte creates a clone, or group of genetically identical cells, some of which are memory cells that will ensure future reactiveness against that antigen.o Because the B-cell and T-cell responses differ significantly from this point on in the sequence, they are summarized separately. T-cell types and responses are extremely varies. When activated (sensitized) by