1. External and Internal Defenses
Immune System Chapter 35
External and Internal
Defenses

2. Phagocytic Cells = cells that engulf & digest antigens
Antigens =any foreign molecule recognized by a
lymphocyte (B or T cell)
a) most are proteins or polysaccharides
b) may protrude from microbe membrane
c) epitope = tiny part the lymphocyte binds

3. MHC = major histocompatibility complex
Set of genes that code for MHC molecules
All are cell surface proteins
Class I MHC display foreign peptides synthesized within the cell (cancer/virus)
found on most body cells
Class II MHC display peptides broken off of microbes during phagocytosis
found on phagocytic cells & B cells

4. * Helper–T cells
Type of T cell that binds to antigens displayed by phagocytic cells or B-cells
(Class II MHC molecules)
Helps promote acquired immune response
Promote Humoral (B-cell) response by secreting cytokines
Promote cell mediated-response (cytotoxic-T cells)

5. External Defenses 1) Skin…waterproof layer dead cells
2) Linings of body tubes/cavities
a) sticky mucus w/ lysozyme enzymes
b) ciliated cells in resp. tract
c) stomach acid & bile salts
d) urine & vaginal low pH
3) Neutral/beneficial bacteria population

6. Internal Defenses I. innate immunity =
present at birth, always present
II. aquired/adaptive immunity =
activated by microbes/antigens
a) Require self-recognition
(membrane proteins)
b) only present in Vertebrates

7. I. Innate Immunity A) Antimicrobial proteins B) inflammatory response
C) Systemic response
D) Phagocytic Cells
E) Natural Killer Cells

8. A) Antimicrobial Proteins
1) interferon ά and β
secreted by virus-infected cells
help neighboring cells inhibit viral reproduction
2) complement system = 30 serum proteins
a. microbe presence activates complement proteins
b. cascade of chem rxns lead to
c. microbe lysis
d. plays a role in inflammation
(complement syst also activated by acquired response)

9. B) Inflammation Triggered by chem. signals: histamine & cytokine
Complement system, allergen, antigen or injury cause mast cells to release histamine
Histamine dilates capillaries, lets out more clotting elements, anti-microbial proteins
Chemokines released by capillary attract phagocytes (macrophages & neutrophiles)
Macrophages secrete cytokines that promote blood flow
Clotting blocks spread of microbes

11. C) Systemic Response Widespread response Increased WBC production
Fever – facilitates phagocytosis
Septic shock – bacterial infection causes high fever low bp may cause death

12. D) Phagocytic cells (4 types)
1) neutrophils – most abundant (1st responders)
2) macrophages – largest
3) eosinophils – poison multi-cellular parasites
4) dentritic cells – activate acquired immunity
a) digest pathogen into pieces
b) bind pathogen pieces to MHC receptors*
c) display pathogen on cell membrane
d) attract helper T cells*
e) helper T cells activate acquired imm. resp.

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14. Natural Killer Cells NK cells vertebrate innate defense.
Attack virus infected cells and cancer cells
NOT MHC molecule receptors…just detects changes in cell membrane
Produce signal molecules that cause apoptosis

15. II aquired/adaptive immunity
A) Humoral Response =
B cells make antibodies
B) Cell Mediated Response =
Cytotoxic T cells defends against:
infected cells
Cancer cells
Transplanted cells

16. Antibodies Proteins secreted by differentiated B-cells (plasma cells)
Bind to specific antigen
Also called Immunoglobulins (Ig)
1) IgG – give passive immunity to fetus
2) IgA – in secretions, sweat, milk, tears
3) IgE – triggers histamines/ allergic rxns

17. Humoral Response = B cells put antibodies in fluid (humor)
1. B cell receptor binds to a Polysaccharide or protein on microbe, pollen or transplanted cell
2. B cell takes in antigen and presents it on MHC surface protein to attract helper T cells
3. Helper T binds to MHC-antigen complex and begins secreting cytokines
4. cytokines induce mitosis in B cell to produce
a. plasma cells
b. memory B-cells

18. Plasma cells make antibodies specific to the antigen that their parent B-cell bound
1.bind & clump viruses/bacteria
2. percipitate antigens dissolved in fluid
3. activate complement proteins
Memory B-cells stay in blood stream ready to become activated in secondary immune response.

19. Secondary Immune Response

20. Cell Mediated Response =fights cancer, infected cells, transplant cells
1. Class I MHC molecule displays antigens
that are synthesized inside affected cell
2. Cytotoxic Tcells bind to MHC-antigen complex (CD8 protein ropes them together)
3. Binding makes cytotoxic Tcell secrete:
a. perforin
(protein, makes holes in membrane)
b. enzymes to digest the cell
c. signals to cause apoptosis

21. More on Helper T Cells Helper T cells Activate by
1. binding C-II MHC-antigen complex
on dendritic cells, macrophages, or B-cells
2. Makes cytokines to stimulate
a. cytotoxic T cells to become active
b. B cells to begin clonal selection…..
mitosis to produce plasma & memory cells

22. Helper T clonal selection
Activated helper T does mitosis
1 clone divides to make many activated helper T cells
The other divides to make memory helper T cells

23. Lymphocyte receptor diversity
1 million diff. B cells….10 million diff. T cells
When leukocytes differentiate into lymphocytes
1. Recombinase enzymes link 1 V gene
segment to 1 J gene segment
2. DNA between selected segments is deleted
3. receptor gene made of V seg + J seg + an
intron and a constant segment.
4. that protein makes the receptors light chain
5. all receptors on that cell the same

25. Autoimmune disease
New lymphocytes are tested to be sure their receptors won’t bind to bodies own cells.
Failure to remove self-reactive cells = autoimmune disease