Lecture 9
Dr. Ismail I. Daood
Medical Microbiology
Immunity, Immunization and Infection :
Immunity terms the ability of a host to resist foreign invaders
(pathogenic microorganism) which divided into two :
1. Non-specific resistance (innate immunity)
2. Specific resistance (acquired immunity).
A. Non-specific resistance : Innate Immunity
Innate immunity is resistance that is not acquired through contact
with a nonself (foreign) entity known as an antigen. It is
nonspecific and includes barriers to infectious agents—e.g, skin
and
mucous
membranes,
phagocytic
cells,
inflammatory
mediators, and complement components. It may vary with age and
with hormonal or metabolic activity
1. mechanical barriers :
a. skin and mucous membrane epithelial
b. secretions (tears, saliva, sweat gland)
c. mucosal secretions in respiratory tract.
d. stomach acidity and intestinal prolytic
e. urinary flow
2. fixed lymph node barriers, e.g., phagocytes, white B.C.
3. blood barrier
4. tissue barrier (biochemical in tissue).
B. Acquired immunity (specific immunity):Adaptive Immunity
Adaptive immunity, which occurs after exposure to an antigen
(e.g., an infectious agent) is specific and is mediated by either
antibody or lymphoid cells. It can be passive or active.
1. humoral immune response by immunoglobulins (Igs) type :
IgG, IgM, IgA, IgE, IgD.
2. cellular immune response (cell-mediated imm.) which in two a ways :
a. actively acquired immunity.
b. passive acquired immunity
Antigens
A substance that reacts with the products of a specific immune response.
The features of antigens that largely determine immunogenicity in the
immune response are as follows.
Foreignness (Difference from "Self")
Molecular Size
Chemical and Structural Complexity
Antigenic Determinants (Epitopes)
Genetic Constitution of the Host
Dosage, Route, and Timing of Antigen Administration
Antibodies
Antibodies (immunoglobulins) are formed by B lymphocytes.
Each individual has a large pool of different B lymphocytes
(about 1011) that have a life span of days or weeks and are found
in the bone marrow, lymph nodes, and gut-associated lymphoid
tissues (eg, tonsils or appendix). These immunoglobulins serve as
receptors for a specific antigen, so that each B cell can respond to
only one antigen or a closely related group of antigens.. B cells
also have surface receptors for the Fc portion of immunoglobulins
and for several complement componentsindividual.
Antibody molecule always consists of identical H chains and identical
L chains. The simplest antibody molecule has a Y shape and consists of
four polypeptide chains: two H chains and two L chains. The four chains
are covalently linked by disulfide bonds.
Immunoglobulin Classes:
Cellular Basis of the Immune Response
The capacity to respond to immunologic stimuli
lymphoid cells. In liver and bone marrow,
differentiate into cells of the red cell series or
lymphoid series. Lymphoid stem cells evolve
lymphocyte populations, B cells and T cells.
resides mainly in
stem cells may
into cells of the
into two main
B Cells
B cells are lymphocytes that develop in the bone marrow in
mammals. In birds they develop in the bursa of Fabricius, a gut
appendage. They rearrange their immunoglobulin genes and
express a unique receptor for antigen on their cell surface
T Cells
T cells are lymphocytes that require maturation in the thymus and
form several subclasses with specific functions. They are the
source of cell-mediated immunity, discussed below
Phagocytosis
During bacterial infection, the number of circulating phagocytic
cells often increases. The main functions of phagocytic cells
include migration, Chemotaxis, ingestion, and microbial killing.
Microorganisms (and other particles) that enter the lymphatics,
lung, bone marrow, or bloodstream are engulfed by any of a
variety of phagocytic cells. Among them are polymorphonuclear
leukocytes (granulocytes), phagocytic monocytes (macrophages),
and fixed macrophages of the reticuloendothelial system.
Factors Affecting Phagocytosis
Phagocytosis is made more efficient by the presence of
antibodies (opsonin) that coat the surface of bacteria and facilitate
their ingestion by phagocytes.
Opsonization can occur by three mechanisms:
(1) Antibody alone can act as opsonin.
(2) antibody plus antigen can activate complement via the
classic pathway to yield opsonin.
(3) opsonin may be produced by a heat-labile system in which
immunoglobulin or other factors activate C3 via the alternative
pathway.
Macrophages have receptors on their membranes for the Fc portion of
antibody and for the C3 component of complement. These receptors aid
the Phagocytosis of antibody-coated particles.
Immunizations :
Immunizations known as vaccination which used vaccines from
whole organisms. Types of vaccines :
1.
killed
(inactivated)
vaccines,
cholera, polio, influenza vaccine).
(e.g.,
typhoid,
2.
attenuated (mutant) vaccines (e.g. new typhoid,
whooping cough, mumps, measeles, rubella, and
chicken pox vaccines).
3.
component vaccines (e.g., polysaccharide, toxoids,
part of cellular as : clostridium, HBV-HBsAg.
4.
recombinant vaccines (e.g., used DNA or RNA as
plasmid
5.
synthetic oligopeptid vaccines ] Experimentally
6.
DNA vaccines
] vaccines