high, but given the pentameric nature, it possesses high avidity of binding. With

time, the level of the IgM decreases, while a concomitant increase in the level an

antibody molecule that possesses much higher affinity for antigen recognition. This

type of antibody is the IgG, with heavy chains of the γ-type.

3.3.1.3

Antigens and Epitopes

An antigen is anything that elicits the production of antibodies whereas an epitope is

the region of the antigen that is recognized by the antibody. Consequently, one

antigen can have many epitopes.

This is an important distinction, since a specific antigen can be recognized by

multiple antibodies; each antibody recognizing the same or a different region of the

molecule (called epitopes). Epitopes have distinct requirements for recognition by

an antibody. For example, linear epitopes that are made up of an amino acid chain,

will be recognized by the antibody even if the protein is not in its native con-

formation. However, in the case of discontinuous epitopes, antibodies recognize a

protein conformation that is a result of folding of an amino acid chain.

Upon recognition of an antigen, antibodies can perform several effector functions.

3.3.1.4

Antibody Effector Functions

a. Complement cascade

One such effector function is the activation of a proteolytic cascade that is shown in

Figure 3.9. Once an antibody recognizes its target on a pathogen surface, C1, a

complement component recognizes the antigen-antibody complex and gets acti-

vated. This sets off a series of proteolytic events, which are amplified in successive

steps. The activated C1 cleaves C2 into C2a and C2b, and cleaves C4 into C4a and

C4b. C2b and C4b combine to form C3 convertase, a protease that cleaves C3 into

C3a and C3b. C3b, also known as C5 convertase cleaves C5 into C5a and C5b. C5b

reacts with other complement components, including C6, C7, C8, and C9 to form a

membrane attack complex. The membrane attack complex forms a hole in the cell

membrane, causing cells to lyse.

C3a and C5a, two fragments generated during complement activation amplify

the inflammatory response by recruiting more phagocytic cells. C3b also attaches to

the surface of microorganisms, making them more likely to be phagocytosed.

Although the events described previously are known as the classical pathway,

the complement cascade can also be activated by two additional pathways, all

converging on the cleavage of C5. In the classical pathway, C1 becomes activated

when it binds to an antigen-antibody complex, whereas in the case of the alternative

pathway, antigens react with C3b, as low amounts C3a and C3b are constitutively

formed and destroyed. When C3b reacts with the antigen, a series of enzymatic

steps are triggered involving factor B, factor D, and properdin and results in the

formation of an activated complex that cleaves C5. The third pathway is triggered

by lectins that recognize pathogens containing mannose-rich cell walls. This

complex activates two proteases mannose-associated serine proteases 1 and 2

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Bioprocessing of Viral Vaccines