Moreover, many harmless foreign substances enter the body, for example, as food or inhaled material, and it would be pointless and potentially dangerous to mount adaptive immune responses against them. Such inappropriate responses are normally avoided because innate immune responses are required to call adaptive immune responses into play and do so only when the innate cells’ PRRs recognize microbial PAMPs, as we discussed earlier. One can trick the adaptive immune system into responding to a harmless foreign molecule, such as a foreign protein, by co-injecting a molecule (often of microbial origin) called an adjuvant, which activates PRRs. This trick is called immunization and is the basis of vaccination. Any substance capable of stimulating B or T cells to make a specific adaptive immune response against it is referred to as an antigen (antibody generator).
There are two broad classes of adaptive immune responses—antibody responses and T‑cell‑mediated immune responses, and most pathogens induce both classes of responses. In antibody responses, B cells are activated to secrete antibodies, which are proteins that circulate in the bloodstream and permeate the other body fluids, where they can bind specifically to the foreign antigen that stimulated their production (see Figure 24–2). Binding of antibody neutralizes extracellular viruses and microbial toxins (such as tetanus toxin or cholera toxin) by blocking their ability to bind to receptors on host cells. Antibody binding also marks invading pathogens for destruction, both by making it easier for phagocytes of the innate immune system to ingest and destroy them and by activating the complement system.
In T-cell-mediated immune responses, T cells recognize foreign antigens that are bound to MHC proteins on the surface of host cells such as dendritic cells, which are specialized for presenting antigen to T cells and are therefore referred to as professional antigen‑presenting cells (APCs). Because MHC proteins carry fragments of pathogen proteins from inside a host cell to the cell surface, T cells can detect pathogens hiding inside a host cell and either kill the infected cell (see Figure 24–2) or stimulate phagocytes or B cells to help eliminate the pathogens.
In this section, we discuss the origins and general properties of B and T cells. In later sections, we consider the specific properties and functions of these cells.
B cells Develop in the Bone Marrow, T cells in the Thymus
There are about 2 × 1012 lymphocytes in the human body, making the immune system comparable in cell mass to the liver or the brain. They occur in large numbers in the blood and lymph (the colorless fluid in the lymphatic vessels, which connect the lymph nodes in the body to each other and to the bloodstream). They are also concentrated in lymphoid organs, such as the thymus, lymph nodes, and spleen (Figure 24–12), and many are also found in other organs, including skin, lung, and gut.
T cells and B cells derive their names from the organs in which they develop: T cells develop in the thymus, and B cells, in adult mammals, develop in the bone marrow. Both types of cells develop from lymphoid progenitor cells that are produced from multipotent hematopoietic stem cells, which are found mainly in the bone marrow (Figure 24–13). The hematopoietic stem cells give rise to more than just lymphocytes: as discussed in Chapter 22, they produce all of the cells of the
Figure 24–12 Human lymphoid organs.