Whereas innate immune responses are general defense reactions that can involve almost any cell type in an organism, the adaptive immune responses are highly specific to the particular pathogen that induced them and depend on a class of white blood cells (leukocytes) called lymphocytes. There are two major classes of lymphocytes that mount adaptive immune responses—B lymphocytes (B cells), which secrete antibodies that bind specifically to the pathogen, and T lymphocytes (T cells), which can either directly kill cells infected with the pathogen or produce secreted or cell-surface signal proteins that stimulate other host cells to help eliminate the pathogen (Figure 24–2). Unlike innate immune responses, which are generally short-lasting, the adaptive responses provide long-lasting protection: a person who recovers from measles or is vaccinated against it, for example, is protected for life against measles by the adaptive immune system, although not against other common viruses, such as those that cause mumps or chickenpox.
Both the innate and adaptive immune systems have evolved sensing mechanisms that enable them to recognize harmful invaders (pathogens) and distinguish them from both the host’s own cells and molecules and harmless or beneficial foreign organisms and their molecules. The innate system relies on sensor proteins that recognize particular types or patterns of molecules that are common to pathogens but are absent or sequestered in the host. The adaptive system, by contrast, uses unique genetic mechanisms to produce a virtually limitless diversity of related proteins—receptors on T and B cells and secreted antibodies—that, between them, can bind almost any foreign molecule. This remarkable strategy enables the adaptive immune system to react specifically against any pathogen, even if the animal never encountered it before. But, it also requires that the system learn not to react against self molecules or harmless foreign ones; if these learning mechanisms fail, harmful autoimmune or allergic responses result.
In this chapter, we focus on vertebrate immune responses and the features that distinguish them from other kinds of cell responses. We begin with innate immune defenses and then discuss the highly specialized properties of the adaptive immune system.
The INNATe IMMUNe SYSTeM
OVerVIeW OF The ADApTIVe IMMUNe SYSTeM B ceLLS AND
Figure 24–1 Innate and adaptive immune responses. Innate immune responses are activated directly by pathogens and defend all multicellular organisms against infection. In vertebrates, pathogens, together with the innate immune responses they activate, also stimulate adaptive immune responses, which then work together with innate immune responses to help fight the infection.
The INNATe IMMUNe SYSTeM
Adaptive immune responses are slow to develop when a vertebrate first encounters a new pathogen. This is because the specific B cells and T cells that can respond to a particular pathogen are initially few in number and must be stimulated to proliferate and differentiate before they can mount effective adaptive immune responses, which can take days. By contrast, a single bacterium that divides every hour can generate almost twenty million progeny in a single day, producing a full-blown infection. Vertebrates, therefore, rely on their innate immune system to defend them against infection during the first critical hours and days of exposure to a new pathogen. Plants and invertebrates lack adaptive immune systems and therefore rely entirely on innate immunity for protection against pathogens.
In this section, we consider some of the strategies the innate immune system uses to recognize pathogens and to provide a first line of defense against them.
epithelial Surfaces Serve as Barriers to Infection