Porth's Pathophysiology, 9e

Chapter 13 Innate and Adaptive Immunity    297

Activated B cell

Antigen

Variable region (heavy chain)

Activated B cell

Fab

Constant region (heavy chain)

Memory B cell

Naive B cell

Primary response

Secondary response

Variable region (light chain)

8

12

10

4

6

2

Weeks

Constant region (light chain)

Antigen

Antigen

FIGURE 13.10  •  Primary and secondary or memory phases of the humoral immune response to the same antigen.

Fc

­antibody occurs sooner and reaches a higher level because of available memory cells. During the primary response, B lymphocytes proliferate and differentiate into antibody-secreting plasma cells. A frac- tion of the activated B cells do not undergo differentiation but rather remain intact to form a pool of memory B lymphocytes that then become available to efficiently respond to invasion during subsequent exposure. Activated T cells can also gener- ate primary and secondary cell-mediated immune responses and the concurrent development of T memory cells. The immunization process makes use of the primary and secondary immune responses. The initial vaccination causes production of both plasma cells and memory cells. The plasma cells destroy the invading organism or toxin, and the mem- ory cells provide defense against future exposure. “Booster” immunizations produce an immediate ­antigen–­antibody response that simulates an immediate rise in antibody levels. Current phase I clinical immunization trials for cancer treat- ment show dense concentrations of CD4 + and CD8 + T lym- phocytes and plasma cells in preexisting tumors after vaccina- tion with irradiated malignant cells. 32 T lymphocytes serve many functions in the immune system including the activation of other T cells and B cells, control of intracellular viral infections, rejection of foreign tissue grafts, activation of autoimmune processes, and activation of delayed hypersensitivity reactions. These processes make up the body’s cell-mediated or cellular immunity . The effector phase of cell-mediated immunity is carried out by T lympho- cytes and macrophages. T lymphocytes arise from lymphoid stem cells in the bone marrow, but unlike B lymphocytes, they migrate to the thymus gland to undergo the process of maturation. The thy- mus gland is richly innervated and produces several ­peptide T Lymphocytes and Cellular Immunity

Heavy chain

FIGURE 13.9  •  Schematic model of an immunoglobulin G (IgG) ­molecule showing the constant and variable regions of the light and heavy chains.

Humoral Immunity Humoral immunity requires the presence of mature B lym- phocytes capable of recognizing antigen and which can ultimately mature into antibody-secreting plasma cells. The ultimate response of the antigen–antibody complex formation can take several forms including antigen–antibody complex precipitation, agglutination of pathogens, neutralization of toxins, phagocytosis or lysis of invading organisms, immune cell activation, and complement activation. Two separate but interrelated responses occur in the development of humoral immunity: a primary and a secondary response (Fig. 13.10). A primary immune response develops when the body encounters the antigen for the first time. The antigen comes in contact with various APCs including mac- rophages, DCs, and B lymphocytes. The antigen is processed by these cells in association with the MHC-II molecules on the cells surface and then presented to the lymphocytes ( i.e., CD4 + T-helper cells) to initiate the immune process. APCs such as macrophages also secrete ILs, which are essential for CD4 + helper T cell activation. 31 The activated CD4 + helper T cells trigger B cells to proliferate and differentiate into clone plasma cells that produce antibody. The primary immune response takes 1 to 2 weeks, but once generated, detectable antibody continues to rise for several more weeks even though the infectious process has resolved. The memory phase or secondary immune response occurs on subsequent exposure to the antigen. During the secondary response, the rise in