Porth's Essentials of Pathophysiology, 4e

336

Infection and Immunity

U N I T 4

B-cell antigen receptor on surface of immunoglobulin

Memory B cell

B cell

B cell

Antigen MHC-II

Proliferation

Differentiation

TCR

Plasma cell

Helper T cell

CD4

Cytokines

B cell

TCR

Antibody

FIGURE 15-9. Pathway for B-cell differentiation. MHC, major histocompatibility class;TCR,T-cell receptor.

Immunoglobulins Immunoglobulins, or antibodies, function as antigen receptors for B cells or as effector molecules of the humoral immune response. The immunoglobulins have been divided into five classes—IgG, IgA, IgM, IgD, and

IgE—each with a different role in the immune defense strategy (Table 15-4). Immunoglobulins have a charac- teristic four-polypeptide structure consisting of at least two identical antigen-binding sites (Fig. 15-10). Each Ig is composed of two identical light (L) chains and two identical heavy (H) chains to form a “Y”-shaped

TABLE 15-4 Classes and Characteristics of Immunoglobulins Figure Class Percentage of  Total

Characteristics

IgG

75.0

Displays antiviral, antitoxin, and antibacterial properties; only Ig that crosses the placenta and thus responsible for protection of newborn; activates complement and binds to macrophages; prominent in the secondary immune response

IgA

15.0

Predominant Ig in body secretions, such as saliva, nasal and respiratory secretions, and breast milk; protects mucous membranes

IgM

10.0

Forms the natural antibodies such as those for ABO blood antigens; prominent in early immune responses; activates complement

IgD

0.2

Found on B lymphocytes; needed for maturation of B cells

IgE

0.004

Binds to mast cells and basophils; involved in parasitic infections and allergic and hypersensitivity reactions