The main element of the programming portion of the immune system is the macrophage. A common property of macrophages is phagocytosis, the capacity to engulf a particle or cell through invagination and sealing off of the cell membrane. The macrophages involved in the immune response set in motion a unique amplification process, so that a large response is obtained relative to the amount of antigen processed. The macrophages engulf antigenic particles and incorporate them into their cytoplasm, where the antigens are fragmented. The fragments are then combined with MHC-II, displayed on the cell membrane of the macrophage, and presented to the immune system. The presented antigens interact with B cells, causing differentiation to plasma cells and Ab secretion. T-helper cells also interact with the presented antigen and are stimulated to cause the B cells to proliferate and mature. Plasma cells are monoclonal (genetically identical) and produce monoclonal Ab. The process, from the pluripo-tent stem cell, to the B and T cells, to the plasma cells, is shown in Figure 5.1.
Figure 5.1 also indicates the actual Ab-producing steps. Plasma cells are clones of Ab-producing cells, which amplify the Ab response by their sheer numbers. The plasma cells can easily be regenerated if called on to do so by the memory functions. A population of plasma cells is shown at the bottom of the diagram. These are identical and amplify and produce large quantities of Ab, proportionally much greater than the amount of antigen that was initially processed.
Because the programmed immune system has the property of memory, subsequent exposures to the same antigen are immediately countered. The actual memory response is referred to as the anamnestic response, a secondary response of high Ab titer to a particular antigen. This is due to "memory cell" formation as a result of the initial antigen stimulus (sensitization or immunization). The anamnestic response is demonstrated in Figure 5.7.
An Ab is bivalent, and an antigen is multivalent, so lattice formation can occur (Fig. 5.8). The complex may be fibrous, particulate, matrixlike, soluble, or insoluble. These characteristics dictate the means of its disposal. Four fundamental reactions describe these processes: neutralization, precipitation, agglutination, and bacteriolysis.
Neutralization is an immunological disposal reaction for bacteria and for toxins (which are small and soluble). Once they bind the Ab, they are no longer toxic because their active site structures are covered and they cannot bind their
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