BSC 310 Lecture Notes - Lecture 37: Immunoglobulin D, Anaphylaxis, Immunoglobulin A
Antibody Mediated (Humoral) Immunity (AMI)‐
On exposure to antigenic determinants in lymphatic organs, B lymphocytes are ‐
activated and differentiated to form plasma cells. Plasma cells are specialized,
differentiated cells that synthesize and secrete antibodies specific for an antigen. Other
activated B lymphocytes form‐ memory cells.These cells can be activated later to
differentiate to plasma cells for rapid antibody production. This antibody production will
occur on future reentry of the antigen to the body and is the basis of long term ‐
immunity.
The products of plasma cells are antibodies. An antibody is a specialized protein substance
produced by the host cells in response to an antigen in the host's tissues. Antibodies are
capable of reacting specifically with the antigen that provoked their production. Antibodies
are often referred to as immunoglobulins. They circulate in the blood and are associated with
the gamma globulin fraction of the plasma.
Structure of antibodies. Structurally, the antibodies (immunoglobulins) are composed of four
protein chains arranged in a distinctive pattern. Each molecule has two long chains of heavy
molecular weight protein (H chains) and two short chains of light molecular weight protein (L
chains). The chains are linked by sulfur bonds.
At the outer end of each arm of the antibody molecule, a specific amino acid sequence
exists. This is where the antibody molecule reacts with the antigenic determinant that
provoked its production. The combining site is known as the Fab region. The most common
antibody molecules have two Fab regions and are said to be bivalent (having two combining
sites). The remaining portion of the antibody molecule is called the Fc region because it can
be crystallized. Its amino acid content is relatively constant and characteristic for its class.
This portion of the molecule activates the complement system and encourages
phagocytosis.
When the antibody molecule reacts with the antigen, the two surfaces fit together like the
pieces of a jigsaw puzzle. This “recognition” is exquisitely accurate and accounts for the
extreme specificity of antibody molecules.
Classes of antibodies. Five classes of antibodies (immunoglobulins) are produced by the
plasma cells. The first class, called IgM, is the major component of the primary antibody
response in adult humans and is the first antibody to appear in the immune reaction. It is
composed of five units joined by a J (joining) chain (Figure 1 ). IgM cannot diffuse through
cell membranes and is found almost exclusively in the blood. Because of its many binding
sites, it is more reactive with antigens than are other kinds of antibodies. IgM is also formed
by the fetus during uterine development and is active against the A and B factors of the red
blood cells. Many antitoxins formed against bacterial toxins are composed of JgM.
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Document Summary
On exposure to antigenic determinants in lymphatic organs, b lymphocytes are activated and differentiated to form plasma cells. Plasma cells are specialized, differentiated cells that synthesize and secrete antibodies specific for an antigen. Other activated b lymphocytes form differentiate to plasma cells for rapid antibody production. This antibody production will occur on future reentry of the antigen to the body and is the basis of long term immunity. memory cells. these cells can be activated later to. An antibody is a specialized protein substance produced by the host cells in response to an antigen in the host"s tissues. Antibodies are capable of reacting specifically with the antigen that provoked their production. They circulate in the blood and are associated with the gamma globulin fraction of the plasma. Structurally, the antibodies (immunoglobulins) are composed of four protein chains arranged in a distinctive pattern.
