Lecture #13: The Immune System
I. NONSPECIFIC DEFENSE MECHANISMS (Fig. 43.1)
1. First line of defense:
a. the skin: tough, dense collagen is a barrier to bacteria and viruses.
b. secretions of the skin:
(1) oil and sweat glands with a pH of 3-5 kills many microbes.
(2) sweat has lysozyme, an antimicrobial enzyme that attacks cell walls of bacteria.
c. mucous membranes: (especially in respiratory tract, and other entrances to body)
(1) nostril hairs and membranes do preliminary trapping of microbes and particles.
(2) mucus secretions along epithelial linings further trap microbes.
(3) ciliated epithelial cells sweep out microbes and particles.
2. Second line of defense:
a. phagocytic white cells:
(1) neutrophils (60-70% of white cells) phagocytose microbes (but live a few days).
(2) monocytes (5% of WBCs) migrate into tissue and form macrophages (lasting)
(3) eosinophils (1.5% of WBCs) have destructive enzymes against larger parasites.
(4) cytotoxic (natural killer) cells destroy body's own virus-infected cells and tumor cells.
b. antimicrobial proteins:
(1) lysozyme, mentioned above, is an anti-microbial protein.
(2) interferons, are produced by virus infected cells (cold & flu) to stimulate the defense mechanisms (that inhibit viral replication) in local uninfected cells.
(3) complement is a group of 20 proteins that cause lysis of microbes.
c. an inflammatory response: (manifested by hyperemia and edema) (Fig. 43.5)
(1) histamine is released from basophils and mast cells to cause dilation & edema.
(2) prostaglandins cause vasodilation and smooth muscle contraction.
(3) chemotaxic substances attract leukocytes into the area.
II. SPECIFIC DEFENSE MECHANISM; SOME BASIC
CONCEPTS
1. The immune system: (Fig. 43.4)
2. Antigens are any foreign substances that elicit an immune response: (Fig. 43.14)
a. includes molecules of viruses, bacteria, fungi, protozoans and parasitic worms.
b. include surface molecules on pollen, insect venom, and transplanted tissues.
c. epitopes (antigenic determinants) are parts of molecules that react.
3. Antibodies are large proteins (IGs) produced by lymphocytes to counter antigens.
a. a Y-shaped protein with two heavy chains and two light chains (Fig. 43.15)
b. these identical chains are bound together by disulfide bridges.
c. carbohydrate (CHO) groups are important in determining molecule's reactivity.
d. there are five major classes of antibodies (IGs) (Table 43.1).
e. the constant part of the molecule is the same in all antibodies of same class.
f. the variable part has the binding sites that react with antigen.
4. Mechanisms of antibody action (Fig 43.16)
a. neutralization = blockage of viral binding sites, or coating of bacterial toxins.
b. agglutination = clumping by cross-linking adjacent antigens (easier to ingest).
c. precipitation = cross-linking of soluble antigen molecules (easier to ingest).
d. complement activation = presence of antibodies on the surface of foreign cells cause lesions in the foreign cell's membrane and cause cells to lyse. (Fig. 43.17)
III. OTHER CHARACTERISTICS OF THE IMMUNE SYSTEM:
1. Specificity refers to the systems ability to recognize particular microorganisms and foreign molecules by producing antigen-specific antibodies.
2. Diversity refers to the immune systems ability to respond to millions of different kinds of antigens.
3. Self/nonself recognition refers to the immune systems ability to distinguish the body's own molecules from foreign molecules.
4. Memory refers to the systems ability to respond more rapidly and effectively to antigens that it has been exposed to before.
5. Active versus passive immunity: (immunity refers to antibody production)
a. passive immunity is when antibodies cross placenta, or gamma globulin given.
b. active immunity is when body's immune system is induced to produce antibodies.
IV. HUMORAL IMMUNITY VERSUS CELL-MEDIATED
IMMUNITY:
1. The development of immunocompetence of B- and T-lymphocytes. (Fig. 43.8)
2. Principal function of B-lymphocytes (B cells): (bone marrow cells)
a. carry out humoral immunity.
b. refers to antibodies circulating in the blood and lymph.
c. defends primarily against free bacteria and viruses in fluids.
3. Principal function of T-lymphocytes (T cells): (thymus cells)
a. carry out cell-mediated immunity.
b. the antibodies remain attached as receptors on specialized cells, mainly T cells.
c. defends primarily against infected host cells, fungi, and protozoa.
(1) cytotoxic-T cells (i.e., killer-T cells)
(2) helper T cells (help both T cells and B cells).