b. Host-Related Factors.
(1) Nonspecific factors. The response to a given immunogen is not only a
function of the physiochemical properties of the substance, but also is influenced by
several host-related factors, including genetic makeup, age, and host environmental
and nutritional status. Existing disease in the host may alter immune response
(2) Antigen dose and administration route. As a rule, low antigen doses
induce the formation of small amounts of antibody with high affinity and specificity. Low
doses injected frequently over long periods of time will induce greater response than
large doses over a short period of time. The route of antigen administration greatly
affects the nature of the immune response.
Section II. ANTIBODIES
Just as antigens are defined in terms of their reactivity with antibodies, all
antibodies are intimately associated with their antigens. Antibodies belong to a group
of proteins called globulins. More specifically, since they are active in immunity, they
are frequently called immunoglobulins. They are a collection of protein molecules
capable of specifically combining with the antigen that caused their formation.
a. Each immunoglobulin is composed of at least one basic unit or monomer,
consisting of four polypeptide chains (Figure 2-1). This basic four-chain subunit
consists of two identical heavy chains (H) and two identical light chains (L). The hinge
region is found at the central junction of Figure 2-1. The chains above the hinge region
yield Fab (antigen-binding) fragments. The chains below the hinge region yield Fc
b. Based on structural differences in the constant regions, there are five classes
of heavy chains. The different forms of heavy chains are designated gamma, alpha,
mu, delta, and epsilon. The type of heavy chain determines the class of the
immunoglobulin. There are five classes of immunoglobulins, designated as IgG, IgM,
IgA, IgD, and IgE. Light chains can be classified as kappa and lambda on the basis of
multiple structural differences in the constant region.