The Basic Adaptive Immune Response
Vertebrates can detect and dispose of most foreign macromolecules. Foreign macromolecules that can stimulate the immune system to produce a response are called antigens. These can include protein, nucleic acids, polysaccharides, and lipids that originate in a vertebrate from bacterial, viral, or parasitic infections, from injections of foreign materials, or from the transformation of a normal cell into a cancer cell. The transformed cells usually possess altered cell surfaces that often are recognized by the immune system.
Evolution has chosen a compromise between the ability of an animal to respond immediately to any one of millions of foreign antigens and the enormous burden that would result from being able to respond quickly. Young vertebrates possess the ability to recognize and respond to most foreign invaders, but this ability begins at a very low level for most antigens. Only some time after exposure to an antigen does the animal’s ability to respond to that antigen begin to be significant. Thereafter, the animal can respond more quickly and more vigorously to the reappearance of a particular antigen. As a result of such exposure animals acquire immunity.
In most cases, the immune system can mobilize to deal with a new antigen before the presence of its source can harm the animal. Speeding and amplifying the immune response is the objective of medical immu-nization. In such immunizations, antigens are injected to prime the immune system. To avoid transmitting the disease associated with the antigen, antigenic material extracted from the organism is injected, or the organism is killed before injection, or a relatively harmless variant is injected. Therefore the animal generates an immune response without suffering the actual dangerous infection. Often, a second or third injection is made after several weeks to boost the animal’s response.
Two responses begin to be apparent within a week of the introduction of a new antigen into an animal. New proteins called antibodies appear in the serum. They are able to bind specifically to the antigen and not to other molecules. Also, a subpopulation of the cell type called T lymphocytes appears. T lymphocytes, which ordinarily are found in the blood, spleen, and lymph nodes, have the ability to bind parts of the antigen to their surface.
In some cases, the mere binding of antibodies or T cells to an antigen is enough to protect the animal from the source of the antigen. As an example, the binding of antibody to the flagella of Salmonella, discussed in the previous chapter, may sufficiently immobilize the cell that it cannot survive in the intestinal tract. Usually, however, an animal’s immune response includes more than just antibody molecules or T cells binding to the antigen. Additional proteins and cell types may partici-pate in elimination of antigens. These recognize complexes of antibody bound to antigen and act to kill, digest, or eliminate the foreign material as well as to modulate the additional synthesis of antibodies and specific T cells. One such array of proteins is called the complement system.
In addition to the T lymphocytes, B lymphocytes also play an impor-tant role in the immune response. They are found in bone marrow, blood, spleen, and lymph nodes where they synthesize and secrete antibody molecules. All the antibody molecules synthesized by a par-ticular B cell are of a single type. These bind to best to antigenspossessing the same shape which elicited their synthesis and less well to molecules possessing similar shapes. Analogously, each T cell has a single binding specificity. One of the problems of immunology is under-standing how the different B or T cells acquire the ability to synthesize proteins with recognition specificities. As we will see in more detail later, different immature lymphocytes possess the ability to recognize different antigens. The presence of one of these antigens can trigger one of the pre-B cell lymphocytes to begin growth and division. This growth requires both the antigen binding to the surface of the B cell and stimulation from a class of T cells that is defined by a surface antigen of its own called CD4. After about twelve divisions of B cells, this expanded clone of identical cells can synthesize and secrete appreciable quantities of antibody.