Porth's Pathophysiology, 9e

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UNIT IV Infection, Inflammation, and Immunity

Understanding

The Complement System (Continued)

Late-Step Membrane Attack In the late-step responses, C3b binds to other complement proteins to form an enzyme that cleaves C5, generating C5a and C5b fragments. C5a stimulates the influx of neutrophils and the vascular phase of acute inflammation. The C5b frag- ment, which remains attached to the microbe, initiates the formation of a complex of complement proteins C6, C7, C8, and C9 into a membrane attack complex protein, or pore, that allows fluids and ions to enter and cause cell lysis.

C5

Membrane attack complex

C5b

C5b

C6,C7,C8,C9

Lysis of microbe

Antigens possess immunologically active sites called antigenic determinants , or epitopes . These are smaller, dis- crete components of the antigen that have a unique molecular shape, which can be recognized by and bound to a specific Ig receptor found on the surface of the lymphocyte or by an antigen-binding site of a secreted antibody (Fig. 13.4). It is not unusual for a single antigen to possess several antigenic determinants and, therefore, be capable of stimulating sev- eral different T and B lymphocytes. For example, different proteins that comprise the influenza virus may function as unique antigens (A, B, C, H, and N antigens), each of which contains several antigenic determinants. Hundreds of anti- genic determinants are found on structures such as the bacte- rial cell wall. Low molecular weight molecules (<10,000 Da) may contain antigenic determinants but alone are usually unable to stimulate an immune response. These molecules are known as haptens . When they are complexed with an immunogenic carrier (usually a protein), they function as antigens. Many haptens exist in nature and frequently create problems for humans. Urushiol is a toxin found in the oils on poison ivy that

Epitopes (antigenic determinants)

Antibody A

Antigen- binding sites

Antigen

Antibody B

Antibody C

FIGURE 13.4  •  Multiple epitopes on a complex antigen being recog- nized by their respective ( A , B , C ) antibodies.