Humoral (antibody-mediated) immunity is directed primarily against (1) exotoxin-mediated diseases such as tetanus and diphtheria, (2) infections in which virulence is related to polysaccharide capsules (e.g., pneumococci, meningococci, Haemophilus influenzae), and (3) certain viral infections. In this chapter, the kinetics of antibody synthesis (i.e., the primary and secondary responses) are described. The functions of the various immunoglobulins are summarized in this chapter and are described in detail in Chapter 59.
THE PRIMARY RESPONSE
The primary response occurs the first time that antigen is encountered. In the primary response, antibodies are detectable in the serum after a longer lag period than occurs in the secondary response. This concept is medically important because the protection afforded by a vaccine the first time it is given is delayed compared with the protection afforded by a booster shot in which a faster secondary response occurs.
The lag period of the primary response is typically 7 to 10 days but can be longer depending on the nature and dose of the antigen and the route of administration (e.g., parenteral or oral). A small clone of B cells and plasma cells specific for the antigen is formed. The serum antibody concentration continues to rise for several weeks, then declines and may drop to very low levels (Figure 60–1). The first antibodies to appear in the primary response are IgM, followed by IgG or IgA. IgM levels decline earlier than IgG levels.
FIGURE 60–1 Antibody synthesis in the primary and secondary responses. In the primary response, immunoglobulin (Ig) M is the first type of antibody to appear. In the secondary response, IgG appears earlier and shows a more rapid rise and a higher final concentration than in the primary response. If at the time of the second exposure to the antigen (Ag1), a second, non–cross-reacting antigen (Ag2) was injected, a primary response to Ag2 would occur while a secondary response to Ag1 was occurring.