1. Innate immunity provides the first line of defense against pathogens and includes anatomical barriers, inflammation, phagocytosis, and antimicrobial proteins/peptides. 2. Adaptive immunity develops over time upon exposure to specific pathogens and provides enhanced protection through antibody production and immunological memory. 3. The major categories of innate immunity defenses are anatomical barriers, inflammation, phagocytosis, microbial antagonism by normal flora, and antimicrobial substances in tissues. Adaptive immunity involves B cells, T cells, and production of antibodies.

1. Immune Defense against
Bacterial Pathogens

4. Immune Defense against Bacterial
Pathogens: Innate Immunity
Innate Immunity is a form of non specific host defense
against invading bacteria. It is natural or “innate” to the
host, depending, in part, on genetics. Innate defense
mechanisms are constitutive to the host, meaning they
are continually ready to respond to invasion and do not
require a period of time for induction.

5. Categories of Innate or Nonspecific Immunity
1. Differences in susceptibility to certain pathogens
2. Anatomical defense
3. Tissue bactericides, including complement
4. Microbial antagonism
5. Inflammation (ability to undergo an inflammatory
response)
6. Phagocytosis
The first four categories are generally considered
non cellular defenses.Inflammation and
Phagocytosis are forms of cellular defense.

6. 1.Differences in Susceptibility of Animal Hosts to Microbial
Pathogens (Natural Immunity)
Natural immunity or resistance is based on the genetics of the
host. There are two aspects:
1) resistance among all members of a species, called species
resistance ；
2) resistance within members of the same animal species,
called individual resistance.

7. Species resistance
1 ） Absence of specific tissue or cellular receptors for
attachment (colonization) by the pathogen.
2)Temperature of the host and ability of pathogen to
grow.
3)Lack of the exact nutritional requirements to support
the growth of the pathogen.
4)Lack of a target site for a microbial toxin.

8. Individual resistance
Age. The changes in immunological system and normal flora .
Sex. It could be due to anatomical structure related to sex , and possibly the
effects of sex hormones on infections.
Stress. The best explanation is that in time of stress the output of cortisone from
the adrenal cortex is increased. This suppresses the inflammatory processes .the
overall effect may be harmful. There are also a number of relationships between
stress-related hormones and the functioning of the immune defenses.
Diet, malnutrition. Infections may be linked with vitamin and protein deficiencies.
Also, overfed and obese animals are more susceptible to infection.
Intercurrent disease or trauma. The normal defenses of an animal are impaired by
organic diseases such as leukemia, Hodgkin's disease, diabetes, AIDS, etc.
Therapy against other diseases. Modern therapeutic procedures used in some
diseases can render an individual more susceptible to infection.

9. Anatomical defenses
associated with tissue
surfaces
2. Anatomical Defenses

10. 3.Microbial Antagonism
There are three main ways that the normal flora protect
the surfaces where they are colonized:
1) Competition with non-indigenous species for binding
(colonization) sites.
2) Specific antagonism against non-indigenous species.
Members of the normal flora may produce very specific
proteins called bacteriocins which kill or inhibit other
(usually closely-related) species of bacteria.
3) Nonspecific antagonism against non-indigenous
species. The normal flora produce a variety of
metabolites and end products that inhibit other
microorganisms. These include fatty acids (lactate,
propionate, etc.), peroxides and antibiotics.

11. Enterococcus faecalis, also classified as Streptococcus faecalis.
Occasionally there is invasion of the host by the normal flora, as evidenced by
this blood culture.

12. 4.Antimicrobial Substances in Host Tissues
Substance Common Sources Chemical
Composition
Activity
Lysozyme Serum, saliva, sweat, tears Protein Bacterial cell lysis
Complement Serum Protein-
carbohydrate
lipoprotein complex
Cell death or lysis of bacteria;
participates in inflammation
histones, ß-
lysins and other
cationic proteins
Serum or organized tissues Proteins or basic
peptides
Disruption of bacterial
plasma membrane
Lactoferrin and
transferrin
Body secretions, serum, organized
tissue spaces
Glycoprotein Inhibit microbial growth by
binding (withholding ) iron
Peroxidase Saliva, tissues, cells
（ neutrophils)
Protein Act with peroxide to cause
lethal oxidations of cells
Fibronectin Serum and mucosal surfaces Glycoprotein Clearance of bacteria
(opsonization)
Interferons Virus-infected cells, lymphocytes Protein Resistance to virus infections
Interleukins Macrophages, lymphocytes Protein Cause fever; promote
activation of immune system

13. The complement cascade, precipitated by certain antigen-antibody reactions
(classical pathway) or by bacterial polysaccharides (alternative pathway), leads to four
principal antimicrobial effects:
1. phagocytes are attracted to the site ; 2. inflammatory agents are produced and/Or
released from cells ; 3. microbes are opsonized to enhance uptake by phagocytic cells
;4. bacteria are lysed in the presence of lysozyme.
Complement

14. 5. Inflammation
Inflammation is necessary for the proper functioning of all the
host defenses, because it focuses all circulating antimicrobial
factors on the site of infection. These include phagocytes,
lymphocytes, antibodies, complement and other antimicrobial
components of plasma.
(1) The inflammatory response is triggered by pathogen invasion
or tissue injury.
(2) The increased acidity activates an extracellular enzyme
kallikrein which in turn activates bradykinin.
(3a) Bradykinin binds to receptors on the capillary walls opening
junctions between cells to allow leakage of plasma components.

16. (3b) Bradykinin also binds to mast cells of the connective tissue that are associated with
the small vessels of most tissues.

17. Phagocytosis by a Macrophage.
MacrophageMacrophage
6.Phagocytic Defenses

18. Immune Defense against Bacterial Pathogens:
Adaptive or Acquired Immunity

19. The immunological System

20. Organs comprising the immune system
Bone Marrow

21. T-cells mature in the thymus

22. Spleen

23. Lymph glands

24. B-cells and T cells

25. T-cells

26. Three important features of an immunological response are:
1. Specificity. An antibody or reactive T-cell will react specifically
with the antigen that induced its formation; it will not react with
other antigens.
2. Memory. The immunological system has a "memory". Once
the immunological response has reacted to produce a specific
type of antibody or reactive T-cell, it is capable of producing more
of the antibody or activated T-cells rapidly and in larger amounts.
3. Tolerance. An animal generally does not undergo an
immunological response to its own components. The animal is
said to be tolerant to self-antigens. Tolerance is brought about in
a number of ways, but basically the immunological system is
able to distinguish "self" antigens from "non-self" antigens.

27. Primary and Secondary Immunological Responses. Following the first exposure to an
antigen the immune response develops gradually over a period of days, reaches a
low plateau within 2-3 weeks, and usually begins to decline in a relatively short period
of time. When the antigen is encountered a second time, a secondary (memory)
response causes a rapid rise in the concentration of antibody, reaching a much higher
level in the serum, which may persist for a relatively long period of time.

28. AMI

33. Exercises:
1. What are the two groups of defense mechanisms of
the body against foreign or harmful agents? What is
the difference between them?
2. What are the categories of innate immunity?
3.What are the important features of an immunological
response?
4.How do antibodies work to prevent the infections?
5. What are the categories of adaptive immunity?