Fish Immunology (1+1)

Unit 8- Antibody (Immunoglobulin)

8.3. Antibody functions
The antibodies have two primary functions:

• they bind to antigens
• they combine with different immunoglobulin receptors specific for them and exert effector functions.

Antigen binding
Immunoglobulins bind specifically to the antigens. Antigen binding by antibodies is the primary function of antibodies and result in protection of the host. If an antibody binds to an antigen on the virus surface then the antibody hinders the virus from binding to the receptor so that the infection cannot be established. Thus, the antibodies neutralize the virus infectivity. Similarly in the case with microbial toxins where antibodies to toxins bind and prevent the induction of biological effects of toxins.
Effector functions
In addition to binding to specific antigen, the antibodies participate in a number of other biological activities known as effector functions. These functions are initiated once the antibody binds to an antigen. The effector functions are mediated by the heavy chain constant regions (Fc regions) of the antibody. Different antibody classes possess different heavy chain constant regions and hence are specialized to perform different effector functions. The various effector functions carried out by antibodies are described below.
Complement activation
Complement refers to a group of serum proteins where the product of one reaction catalyze a second reaction, the product form the second reaction catalyze a third reaction and so on. The complement activation pathway is triggered by antigen-antibody reactions. The Fc region of antibody is involved in the activation of the first component of complement. Then a series of reactions takes place and form a 'membrane attack complex' (MAC). This MAC is able to attack the membrane of the pathogen and cause the lysis of pathogen by forming a hole on membrane that releases the cell components.
Opsonization
Opsonization is a process where macrophages and neutrophiis are involved. These phagocytes carry Fc receptors on their surfaces and once the pathogen is coated with antibody molecules, it encances the opsonizing activity because of the Fc region of antibody, the phagocytes bind the antibodies through Fc receptors, which help them to phagocytose the target organism. Once the phathgen is engulfed it is killed inside the macrophage.
Antibody-dependent cell-mediated cytotoxicity (ADCC)
Natural Killer (NK) cells present in fish also possess Fc receptors. Antibodies after binding to the target cell (e.g., virus infected cell) activate the NK cells to kill the target cell by extracellular mechanism. This process is known as antibody-dependent cell-mediated cytotoxicity.
Transcytosis
Transcytosis is a process by which some antibodies can cross the epithelial layer to reach the mucosal surfaces of the respiratory, gastro–intestinal tract and can bind antigens. The process of transcytosis depends on the properties of constant region of antibody molecules. IgA is the major antibody that can undergo transcytosis.
IgE-mediated function
Mast cells and Basophils (which are rarely seen in fish) contain Fc receptors for IgE antibodies. When IgE binds with antigen the Fc region induces these cells to degranulate. These granular contents initiates an inflammatory response by attracting various molecular and cellular immune effectors which intern destroy the pathogen. This type of defence is effective particularly against parasitic infection.