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21. 3 The Adaptive Immune Response: T lymphocytes and Their Functional Types
21.3 OBJECTIVES
1. List the various characteristics of an antigen
Innate immune responses (and early in- duced responses) are in many cases ineffec- tive at completely controlling pathogen growth. However, they slow pathogen growth and allow time for the adaptive im- mune response to strengthen and either control or eliminate the pathogen. The in- nate immune system also sends signals to the cells of the adaptive immune system, guiding them in how to attack the patho- gen. Thus, these are the two important arms of the immune response.
The specificity of the adaptive immune re-
sponse—its ability to specifically recognize
and make a response against a wide variety
of pathogens—is its great strength. Antigens, the small chemical groups often associated with pathogens, are recognized by receptors on the surface of B and T lymphocytes. The adaptive immune response to these antigens is so versatile that it can respond to nearly any pathogen.
Antigens on pathogens are usually large and complex, and consist of many antigenic deter- minants. An antigenic determinant (epitope) is one of the small regions within an antigen to which a receptor can bind, and antigenic determinants are limited by the size of the re- ceptor itself.
They usually consist of six or fewer amino acid residues in a protein, or one or two sugar moieties in a carbohydrate antigen. Antigenic determinants on a carbohydrate antigen are usually less diverse than on a protein antigen. Carbohydrate antigens are found on bacte- rial cell walls and on red blood cells (the ABO blood group antigens).
Protein antigens are complex because of the variety of three-dimensional shapes that pro- teins can assume, and are especially important for the immune responses to viruses and worm parasites. It is the interaction of the shape of the antigen and the complementary shape of the amino acids of the antigen-binding site that accounts for the chemical basis of specificity
MOVIE 1.34 Professional Anti- gen Presenting Cells 11:32 minutes Khan Academy
Aging and the Immune System
By the year 2050, 25 percent of the population of the United States will be 60 years of age or older. The CDC estimates that 80 percent of those 60 years and older have one or more chronic disease associated with deficiencies of the immune systems. This loss of immune function with age is called immunosenescence. To treat this growing population, medical professionals must better understand the aging process. One major cause of age-related immune deficiencies is thymic involution, the shrinking of the thymus gland that begins at birth, at a rate of about three percent tissue loss per year, and continues until 35–45 years of age, when he rate declines to about one percent loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. Thus, this age is a theoretical limit to a healthy human lifespan. Thymic involution has been observed in all vertebrate species that have a thymus gland. Animal studies have shown that transplanted thymic grafts between inbred strains of mice invo- luted according to the age of the donor and not of the recipient, implying the process is genetically programmed. There is evidence that the thymic microenvironment, so vital to the development of na.ve T cells, loses thymic epithelial cells according to the decreasing expression of the FOXN1 gene with age. It is also known that thymic involution can be al- tered by hormone levels. Sex hormones such as estrogen and testosterone enhance involu- tion, and the hormonal changes in pregnant women cause a temporary thymic involution that reverses itself, when the size of the thymus and its hormone levels return to normal, usually after lactation ceases. What does all this tell us? Can we reverse immunosenes- cence, or at least slow it down? The potential is there for using thymic transplants from younger donors to keep thymic output of na.ve T cells high. Gene therapies that target gene expression are also seen as future possibilities. The more we learn through immunose nescence research, the more opportunities there will be to develop therapies, even though these therapies will likely take decades to develop. The ultimate goal is for everyone to live and be healthy longer, but there may be limits to immortality imposed by our genes and hormones.
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State of Alaska EMS Education Primer - 2016
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