SOLUTIONS TO EXERCISES, LESSON 2
An antigen is traditionally defined as any substance that will cause production of
antibodies and which reacts specifically with those antigens. This term, however,
is incomplete because it emphasizes the production of immunoglobulins.
Therefore, the term immunogen was introduced to include other biological
processes, such as proliferation of lymphocytes and synthesis of recognition
molecules which can specifically combine with the inducing antigen. Antigens are
macromolecules that possess a high degree of internal chemical complexity. They
are soluble or easily solubilized by phagocytic cells of the animal and are foreign
to the animal. (para 2-1a)
A hapten is defined as a small molecule which by itself cannot stimulate antibody
synthesis but will combine with the antibody once formed. When the hapten is
conjugated to a protein molecule called a carrier, it can elicit an immune response.
The capacity to provoke a specific immune response is called immunogenicity.
The capacity to react with the products of the specific immune response is called
antigenicity. Substances that are immunogenic are always antigenic, but antigens
are not necessarily immunogenic. (para 2-2)
The greater the molecular weight of a substance, the more likely it is to function as
an antigen. Immunogenicity increases with structural complexity. Poor antigens
often consist of molecules that are insoluble in body fluids and which cannot be
converted to a soluble form by tissue enzymes. (para 2-2a)
A substance not recognized as belonging to or being a part of the body is said to
be a foreign substance. This characteristic is foreignness.
Some host-related factors in immunogenicity are called nonspecific factors. These
host-related factors include genetic makeup, age, and host environmental and
nutritional status. Existing disease in the host may alter the capability for immune
response. (para 2-2b(1))
Other host-related factors are the antigen dose and administration route. A
greater immune response can be expected with low doses injected frequently over
short periods of time. (para 2-2b(2))
You've already learned that antigens are defined in terms of their reactivity with
antibodies. Likewise, 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, these proteins are frequently called immunoglobulins.
They are a collection of protein molecules capable of specifically combining with
the antigen that caused their formation. (para 2-3)