Chapter Overview

The most specific host defenses are derived from a dual system of lymphocytes that are genetically programmed to react with foreign substances (antigens) found in microbes and other organisms.
These cells carry glycoprotein receptors that dictate their specificity and reactivity.
B lymphocytes have antibody receptors, T lymphocytes have T-cell receptors, and macrophages have histocompatibility receptors such as MHC and HLA.
B cells and T cells arise in the bone marrow, where they proliferate and develop extreme variations in the expression of receptor genes.
Differentiation of lymphocytes creates billions of genetically different clones that each have a unique specificity for antigen.
The B cells reach final maturity in special bone marrow sites, and the T cells reach final maturity in the thymus gland.
Both types of lymphocytes home (migrate) to separate sites in lymphoid tissue where they serve as a constant source of immune cells primed to respond to their correct antigen.
Antigens are foreign cells, viruses, and molecules that meet a required size and complexity. They are capable of triggering immune reactions by lymphocytes.
The B and T cells react with antigens through a complex series of cooperative events that involve the processing of antigens by macrophages and the assistance of helper T cells and cytokine stimulants (interleukins).
B cells activated by antigen enter the cell cycle and mitosis, giving rise to plasma cells that secrete antibodies (humoral immunity) and long-lived memory cells.
Antibodies have binding sites that affix tightly to an antigen and hold it in place for agglutination, opsonization, complement fixation, and neutralization.
The amount of antibodies increases during the initial contact with antigen and rises rapidly during subsequent exposures due to memory cells ready for immediate reactions.
T cells have various receptors that signal their ability to respond as helper cells, suppressor cells, and cytotoxic cells that kill complex pathogens and cancer cells.
Acquired immunities fall into the categories of natural, artificial, active, and passive.

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