Collage of medicine Anatomy and histology department Dr. Hameda A. Gahzi 9\11\2015



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Collage of medicine Anatomy and histology department Dr.Hameda A.Gahzi

9\11\2015




The immune system and lymphoid system

The body has a system of cells—the immune system—that has the ability to distinguish "self" (the organism's own molecules) from "nonself" (foreign substances). This system has the ability to neutralize or inactivate foreign molecules (such as soluble molecules as well as molecules present in viruses, bacteria, and parasites) and to destroy microorganisms or other cells (such as virus-infected cells, cells of transplanted organs, and cancer cells). On occasion, the immune system of an individual reacts against its own normal body tissues or molecules, causing autoimmune diseases.



Antigens

A molecule that is recognized by cells of the immune system is called an antigen and may elicit a response from these cells. Antigens may consist of soluble molecules (such as proteins, polysaccharides, and nucleoproteins) or molecules belonging to whole cells (bacteria, protozoa, tumor cells, or virus-infected cells). The cells of the immune system do not recognize and react to the whole antigen molecule but instead react to small molecular domains of the antigen known as antigenic determinants or epitopes. The response of the organism to antigens may be called cellular (in which lymphocytes are primarily in charge of eliminating the antigen) or humoral (in which molecules secreted by plasma cells, called antibodies, are primarily responsible for the response). Some epitopes (e.g., polysaccharides of bacterial walls or lipids) usually elicit a humoral response whereas proteins elicit both a cellular and humoral response. More details on cellular and humoral immune responses are provided below.





Antibodies

An antibody is a glycoprotein that interacts specifically with an antigenic determinant. Antibodies belong to the immunoglobulin (Ig) protein family. Free molecules of antibodies are secreted by plasma cells that arise by proliferation and terminal differentiation of clones of B lymphocytes whose receptors recognize and bind specific epitopes. These secreted antibodies either circulate in the plasma and may leave the blood vessels reaching the tissues or are present in the secretion of some epithelia (e.g., of the mammary gland and salivary glands). Other antibodies are not free molecules, but are integral membrane proteins of the surface of lymphocytes. In any case, each antibody combines with the epitope that it specifically recognizes.



There are several classes of antibody molecules but all have a common design: they consist of two identical light chains and two identical heavy chains bound by disulfide bonds and noncovalent forces. The isolated carboxyl-terminal portion of the heavy chain molecules is called the Fc region(Figure ,1). The Fc regions of some immunoglobulins are recognized by receptors present on the membrane of several cell types and for this reason antibodies may bind to the surface of these cells. The first 110 amino acids near the amino-terminal part of the light and heavy chains are very variable among different antibody molecules. Therefore, this region of the molecule is called the variable region. The antigen-binding site of an antibody consists of the variable regions of one heavy and one light chain. Thus, each antibody molecule has two binding sites for antigens, both for the same antigen. The molecules of some of the immunoglobulin classes may form dimers, trimers, or pentamers.



(Figure ,1) Basic structure of an immunoglobulin (antibody).

Two light chains and two heavy chains form an antibody molecule. The chains are linked by disulfide bonds. The variable portions near the NH2 end of the light and heavy chains bind the antigen. The Fc region of the molecule may bind to surface receptors of several cell types.

Classes of Antibodies

The main classes of immunoglobulins in humans are immunoglobulin G (IgG), IgA, IgM, IgE, and IgD (Table 14–1).



Table 14–1. Summary of Classes of Antibodies.




 

lgG

lgM

lgA

lgD

lgE

 











Structure

Monomer

Pentamer

Dimer or trimer with secretory component

Monomer

Monomer

Antibody percentage in the serum

80%

5–10%

10–15%

0.2%

0.002%

Presence in sites other than blood, connective tissue, and lymphoid organs

Fetal circulation in pregnant women

B lymphocyte surface (as a monomer)

Secretions (saliva, milk, tears, etc)

Surface of B lymphocytes

Bound to the surface of mast cells and basophils

Known functions

Activates phagocytosis, neutralizes antigens, protects newborn

First antibodies to be produced in an initial immune response; activates complement

Protects the surfaces of mucosas

Functions as a receptor to antigens triggering initial B cell activation

Participates in allergy and destruction of parasitic worms





IgG is the most abundant class representing 75% of serum immunoglobulins. It is produced in large amounts during immune responses. IgG is the only immunoglobulin that crosses the placental barrier and is transported to the circulatory system of the fetus, protecting the newborn against infections for a certain period of time.

IgA is the main immunoglobulin found in secretions, such as nasal, bronchial, intestinal, and prostatic, as well as in tears, colostrum, saliva, and vaginal fluid. It is present in secretions as a dimer or trimer called secretory IgA, composed of two or three molecules of monomeric IgA united by a polypeptide chain called protein J and combined with another protein, the secretory, or transport, component. Because it is resistant to several enzymes, secretory IgA subsists in the secretions where it provides protection against the proliferation of microorganisms. IgA monomers and protein J are secreted by plasma cells in the lamina propria of the epithelium of the digestive, respiratory, and urinary passages; the secretory component is synthesized by the mucosal epithelial cells and is added to the IgA polymer as it is transported through the epithelial cells.

IgM constitutes about 10% of blood immunoglobulins and usually exists as a pentamer. Together with IgD, it is the major immunoglobulin found on the surface of B lymphocytes. These two classes of immunoglobulins have both membrane-bound and circulating forms. IgM bound to the membrane of a B lymphocyte functions as its specific receptor for antigens. The result of this interaction is the proliferation and further differentiation of B lymphocytes into antibody-secreting plasma cells. Secreted IgM, when bound to antigen, is very effective in activating the complement system.

IgE usually exists as a monomer. As its Fc region has a great affinity for receptors present on the surfaces of mast cells and basophils, it attaches to these cells after being secreted by plasma cells and only small amounts are found in the blood. When IgE molecules present on the surface of mast cells or basophils encounter the antigen that elicited the production of this specific IgE, the antigen–antibody complex triggers the liberation of several biologically active substances, such as histamine, heparin, leukotrienes, and eosinophil-chemotactic factor of anaphylaxis. This characterizes an allergic reaction, which is thus mediated by the binding of cell-bound IgE with the antigens (allergens) that stimulated its production.

Actions of Antibodies

Some antibodies are able to agglutinate cells and to precipitate soluble antigens, thus neutralizing their harmful effects on the body (Figure,2). Although phagocytosis of microorganisms and other particles occurs spontaneously this event is greatly stimulated when they are covered by antibodies produced against them, a phenomenon called opsonization (Figure ,2). Opsonization occurs because macrophages, neutrophils, and eosinophils have receptors for the Fc region of IgG on their surfaces. Antigen–antibody complexes and some antigens activate the complement system, a group of around 20 plasma proteins produced mainly in the liver and activated through a cascade of reactions. One of the most important proteins of this system is the component called C3. To defend the body against foreign molecules or cells, the complement system may (1) stimulate phagocytosis of bacteria or other microorganisms because of opsonization due to the binding of C3 fragments to specific C3 receptors present on the surface of phagocytic cells (Figure ,2).



(2) Induce lysis of microorganisms by acting on their cell membranes (Figure ,2).

Mechanisms of antigen inactivation. (1) Agglutination, in which antibodies bind to antigens, forming aggregates and reducing the amount of free antigens; aggregates may be ingested by phagocytes; (2) opsonization of antigens by complement stimulates their phagocytosis; (3) opsonization of antigens by antibodies stimulates phagocytosis; (4) neutralization, in which the binding of antibody to microorganisms blocks their adhesion to cells and inactivates toxins; (5) cytotoxicity mediated by cells, which involves antibodies adhering to the surface of worms activating cells of the immune system (macrophages and eosinophils) and inducing them to liberate molecules that attack the surface of the animal; (6) complement activation, in which the binding of antibodies to the initial protein of the complement system triggers the complement cascade and causes cell lysis.


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