style="font-size:15px;"> b. NK cells can recognize infected cells directly and indirectly.
c. NK cells are similar to, but differ from, CTLs.
Comic Relief
19. How would you attract the attention of a good-looking phagocyte?
20. How would you describe a macrophage that is wearing sunglasses and a camera?
Answers to Comic Relief:
19. Try complements.
20. Phago-sightseer.
4
IN THIS CHAPTER
The Specialists: Adapting to a Particular Pathogen Challenge
B Cells: Producers of Antibodies
The Humoral (Antibody) Immune Response
Characteristics of Antibodies and Their Diverse Roles in Preventing Infection
Secretory Antibodies: Antibodies That Protect Mucosal Surfaces
Pathogen and Toxin Neutralization by Antibodies
Cytotoxic T Cells, Also Known as Cytotoxic T Lymphocytes (CTLs)
Cytotoxic T Lymphocytes: Critical Defense against Intracellular Pathogens
Antigen Presentation to the Immune System
Processing of Protein Antigens by Dendritic Cells
Interaction between APCs and T Cells: The T-Cell-Dependent Response
Th-(Th1/Th2/Th17)-Cell-Mediated Immunity
Production of Antibodies by B Cells
Links between the Innate and Adaptive Defense Systems
T-Cell-Independent Antibody Responses
Mucosal Immunity: IgA/sIgA Antibodies
Development of the Adaptive Immune System from Infancy to Adulthood
Adaptive Defense Systems in Nonmammals
The Dark Side of the Adaptive Defenses: Autoimmune Disease
Solving Problems in Bacterial Pathogenesis
CHAPTER 4
The Adaptive Defenses
ANTIBODIES AND CYTOTOXIC T CELLS
If ordinary police are confronted with a dangerous situation that they are unable to resolve, they call in the SWAT (special weapons and tactics) team, specialists such as snipers and hostage negotiators, who are trained to target and deal with a specific crisis. When the innate immune system is faced with a prolonged bacterial attack that it cannot handle, the human body uses a similar strategy. It brings in the specialists: antibody-producing B cells and cytotoxic T cells, also called cytotoxic T lymphocytes (CTLs), that are designed to target and kill a specific invading microbe.
The Specialists: Adapting to a Particular Pathogen Challenge
The innate defenses of the body are normally an effective protective force that eliminates invading microbes. Unfortunately, some microbes have developed strategies for evading these innate defenses and surviving in the host. For example, some microbes are able to resist phagocytosis by neutrophils, while others are even able to survive and multiply within phagocytic cells once ingested. To cope with these microbes, the body has evolved a second defense system, the adaptive or acquired immune response, which enables the host to recognize specific pathogens and mount a targeted response against them. In this adaptive immune response, dendritic cells (DCs) internalize the foreign substances that are then processed into smaller pieces and presented on their cell surface to T helper (Th) cells. The Th cells in turn stimulate B cells to produce antibodies that specifically recognize the foreign substance, targeting it for opsonization by phagocytes. The Th cells also stimulate cytotoxic T lymphocytes (CTLs) that recognize and kill infected host cells.
Unlike the innate immune system, which mounts a generalized response against foreign invaders, the adaptive immune system involves specialized cells of the body that change or adapt to target individual invaders in a highly specific way. As such, the adaptive response takes time to adjust and fully develop its specific response, taking up to a week to appear on the scene in full force. However, upon subsequent encounters with the same invader, the specific defenses appear much more rapidly, needing only a day or two to respond. In terms of disease manifestation, this usually means that during the first encounter with a new pathogen the body may have to struggle to overcome the infection and may exhibit symptoms of disease. But, upon future exposure to the same pathogen, the body is prepared to rapidly eliminate the threat and can combat the infection much faster than it did the first time around. Vaccination, which will be covered in greater detail in chapter 17, is a strategy for priming the body with an adaptive immune response against a particular pathogen so as to launch this more rapid response without actually having to endure the first, more unpleasant, encounter.
Understanding how the components
