As mentioned in Section 1-6, there are a large number of cytokines with different functions (see Appendix III). A convenient way to organize the effects of cytokines is by the effector module that each cytokine promotes. Some cytokines tend to promote immunity to intracellular pathogens. One such cytokine is interferon-γ, which acts both by activating phagocytes to more efficiently kill intracellular pathogens and by inducing target tissues to resist intracellular pathogens. This is called type 1 immunity. IFN-γ is produced by some but not all subsets of innate and adaptive lymphocytes, and the subset of ILC making IFN-γ is called ILC1. Other ILC subsets produce cytokines favoring effector modules called type 2 and type 3, which coordinate defense against parasitic and extracellular pathogens, respectively. The modular nature of immune effector functions will be encountered frequently throughout this book. One principle seems to be that activated sensor cells from either the innate or the adaptive immune system can activate different subsets of innate or adaptive lymphocytes that are specialized for amplifying particular effector modules that are directed against different categories of pathogens (Fig. 1.27).
the effector mechanisms of immunity.
Fig. 1.27 Innate and adaptive lymphocyte cells share a variety of functions. the different effector modules are served by both innate and adaptive immune mechanisms. For each of the four major types of innate lymphocytes, there is a corresponding type of t cell with generally similar functional characteristics. each set of innate lymphocyte and t cell exert an effector activity that is broadly directed at a distinct category of pathogen.
1-20 Antibodies protect against extracellular pathogens and their toxic products.
Antibodies are found in plasma—the fluid component of blood—and in extracellular fluids. Because body fluids were once known as humors, immunity mediated by antibodies is known as humoral immunity.
Antibodies are Y-shaped molecules with two identical antigen-binding sites and one constant, or Fc, region. As mentioned in Section 1-9, there are five forms of the constant region of an antibody, known as the antibody classes or isotypes. The constant region determines an antibody’s functional properties—how it will engage with the effector mechanisms that dispose of antigen once it is recognized. Each class carries out its particular function by engaging a distinct set of effector mechanisms. We describe the antibody classes and their actions in Chapters 5 and 10.
The first and most direct way in which antibodies can protect against pathogens or their products is by binding to them and thereby blocking their access to cells that they might infect or destroy (Fig. 1.28, left panels). This is known as neutralization and is important for protection against viruses, which become prevented from entering cells and replicating, and against bacterial toxin and is the form of immunity elicited by most vaccines.
For bacteria, however, binding by antibodies is not sufficient to stop their replication. In this case, the function of the antibody is to enable a phagocytic cell such as a macrophage or a neutrophil to ingest and destroy the bacterium. Many bacteria evade the innate immune system because they have an outer coat that is not recognized by the pattern recognition receptors of phagocytes. However, antigens in the coat can be recognized by antibodies, and phagocytes have receptors, called Fc receptors, that bind the constant region and facilitate phagocytosis of the bacterium (see Fig. 1.28, center panels). The coating of pathogens and foreign particles in this way is known as opsonization.