With the looming prospect of mass immunization against influenza, it’s important to understand how vaccines work. To do this we must have a good understanding of adaptive immune defenses. Today we’ll begin a discussion of the humoral arm of the adaptive immune response – antibodies.
Antibodies are large proteins produced by vertebrates that play important roles in identifying and eliminating foreign objects. The basic structural unit is composed of two heavy chains and two light chains, as shown in this diagram.
Antibodies bind other molecules known as antigens. Binding occurs in a small region near the ends of the heavy and light chain called the hypervariable region (labeled only on one arm in the figure). As the name implies, this region is extremely variable, which is why vertebrates can produce millions of antibodies that can bind many different antigens. The part of the antigen that is recognized by the antibody is known as an epitope.
There are five classes of immunoglobulin—IgA, IgD, IgE, IgG, and IgM—defined by the amino acid sequence of the heavy chain. They have different roles in immune responses; IgG, IgA, and IgM are commonly produced after viral infection.
During the first encounter with a virus, a primary antibody response occurs. IgM antibody appears first, followed by IgA on mucosal surfaces or IgG in the serum. The IgG antibody is the major antibody of the response and is very stable, with a half-life of 7 to 21 days. When an infection occurs with the same or a similar virus, a rapid antibody response occurs that is called the secondary antibody response. The specificity and memory of the antibody response are illustrated in the following graph.
A typical adaptive antibody response is shown as the relative concentration of serum antibodies weeks after injection of an animal with antigen A or a mixture of antigens A and B. Maximal primary response to antigen A occurs in 3 to 4 weeks. When the animal is injected with a mixture of both antigens A and B at 7 weeks, the secondary response to antigen A is more rapid and stronger than the primary response, demonstrating immunological memory. As expected, the primary response to antigen B requires 3 – 4 weeks. Antibody levels (also called antibody titers) decline with time after each immunization, a property known as self-limitation or resolution.
Antibodies are critical for preventing many viral infections, and may also contribute to the resolution of infection. We’ll next explore how antibodies accomplish these diverse activities.