BSC 310 Lecture Notes - Lecture 36: Clonal Selection, Histocompatibility, Phagocytosis
Antigens
Immune responses are directed at a series of foreign substances known as antigens,
also referred to as immunogens. Most antigens are high molecular weight substances,
but low molecular weight substances will also act as antigens if they bind to proteins in
the body. The low molecular weight compound is referred to as a hapten. The resulting
conjugate may induce an immune response directed against the antigen.
The uptake and processing of antigens by macrophages in the tissue is an initial, critical
step in most immune responses. The simple act of taking foreign substances into the
body does not necessarily invoke an immune response because the substances may be
broken down before they are ingested by macrophages.
Antigenic determinants. The chemical groups on the antigen molecules that
determine their immunogenicity are called antigenic determinants, also known
as epitopes. Antigenic determinants may consist of several amino acids of a protein
molecule or several monosaccharide units of a polysaccharide. Each species of living
thing is chemically and antigenically unique because of differences in its proteins,
carbohydrates, and other organic substances.
Types of antigens. Certain types of antigens are distinctive. Autoantigens, for
example, are a person's own self antigens. Alloantigens are antigens found in different
members of the same species (the red blood cell antigens A and B are
examples). Heterophile antigens are identical antigens found in the cells of different
species.
A single organism such as a bacterium may contain a variety of proteins,
carbohydrates, and other materials that provoke immune responses. Antigens found on
the body cell are called somatic antigens. Antigens in the bacterial capsule
are capsular antigens. Antigens of an organism's flagella are known as flagellar
antigens (H antigens). Protein substances such as exotoxins are also antigenic.
Cells of the Immune System
Cells of the immune system are associated with the lymphatic system of the body and
its specialized cells. Lymphocytes of the lymphatic system are derived from stem
cells of the bone marrow. These undifferentiated precursor cells proliferate throughout
life and replenish the mature cells of the immune system.
B-lymphocytes and T-lymphocytes. There are two major pathways for the
differentiation of stem cells into immune cells. Certain of the stem cells produce B-
lymphocytes (B-cells) while other stem cells form T-lymphocytes (T-cells). B-
lymphocytes are so named because in birds, they are formed in the bursa of Fabricius.
The equivalent site in humans has not been identified but is believed to be the bone
marrow. T-lymphocytes undergo their conversion in the thymus gland, an organ in the
neck tissues near the trachea and thyroid gland (Figure 1 ).
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Immune responses are directed at a series of foreign substances known as antigens, also referred to as immunogens. Most antigens are high molecular weight substances, but low molecular weight substances will also act as antigens if they bind to proteins in the body. The low molecular weight compound is referred to as a hapten. The resulting conjugate may induce an immune response directed against the antigen. The uptake and processing of antigens by macrophages in the tissue is an initial, critical step in most immune responses. The simple act of taking foreign substances into the body does not necessarily invoke an immune response because the substances may be broken down before they are ingested by macrophages. The chemical groups on the antigen molecules that determine their immunogenicity are called antigenic determinants, also known as epitopes. Antigenic determinants may consist of several amino acids of a protein molecule or several monosaccharide units of a polysaccharide.
