*4.3.3. The γ-globulins*

The γ-globulin fraction is predominantly composed of immunoglobulins (Ig) of various classes (IgG, IgA, IgM, IgD, and IgE). While in some animal species (cattle and goats) the γ-globulins constitute one overall fraction, in sheep, they may be visualized as two subpeaks: the γ<sup>1</sup> and γ2 subfractions [104]. According to Kaneko [22], immunoglobulins from the γ fraction may migrate as fast or slow, which may be seen in these two subfractions. On the other hand, Vavricka et al. [76] indicated that some classes of immunoglobulins may migrate into the β-γ zone or β-region. Immunoglobulins (or antibodies) have major roles in the immune responses of the body, especially in response to foreign molecules, the so-called antigens. Their primary function is the protection of the host due to specific binding of one or a few closely related antigens in order to mediate their neutralization and elimination [223]. Immunoglobulins are produced by cells of the adaptive immune system, activated B cells and plasma cells, in response to the exposure to antigens [8].

The immunoglobulins are glycoproteins composed of two heavy (H) and two light (L) chains linked by disulfide bridges [223]. According to the structure of the H chain, immunoglobulins are classified into the following classes: IgG, IgM, IgA, IgE, and IgD. The L chain consists of either kappa (κ) or lambda (λ) chain, which indicates the type of immunoglobulins. Based on the structural variations in the variable regions of H or L chains, immunoglobulins can be further divided into subtypes and subclasses. For example, two subclasses of IgG have been identified in cattle (IgG1 and IgG2) [7].

Most viral, bacterial, and toxin antibodies are of the IgG type and are present in all animals. It is the predominant type of immunoglobulins found in the body and has the longest serum half-life. IgE is involved in allergic and anaphylactic reactions, whereas IgA can be found in the secretions of the respiratory, genitourinary, and gastrointestinal tracts [7]. IgM functions be opsonizing antigens for destruction and fixing complement, and usually are associated with the first line of defense [224]. IgD is found in very low concentrations in the serum and has a short half-life. The functions of circulating IgD are not well understood [225].
