Chapter 24 The Immune System

I. Innate Immunity

1. The immune system is the body system of defenses against agents (pathogens) that cause disease.

2. All animals have innate immunity, which is a set of defenses that are active immediately upon infection, and are the same whether or not the pathogen has been encountered previously.

3. Invertebrates rely solely on innate immunity. In vertebrates, innate immunity coexists with the more recently evolved adaptive immunity. In mammals, innate defenses include external barriers like skin and mucous membranes.

4. Most innate immune cells are phagocytes, the two main types are:

(1)  Neutrophils: the most abundant type of white blood cells that circulate in the blood and enter tissues at sites of infection.

(2)  Macrophages: large phagocytes are capable of phagocytosis (eating any bacteria and viruses) they encounter.

5. In vertebrates, internal innate immunity includes proteins that either attack infecting microbes directly or impede their reproduction. Interferons are proteins produced by virus-infected cells that help to limit the cell-to-cell spread of viruses.

6. Additional innate immunity in vertebrates is the complement system, a group of about 30 different proteins that circulate in an inactive form in the blood. The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytes to clear pathogens from an organism. These complement proteins can act with other defense mechanisms, leading to the lysis of invading cells.

II. Adaptive Immunity

1. The adaptive immune system, also called acquired immunity, counters specific invaders. Adaptive immunity is a set of defenses, found only in vertebrates, that is activated in response to specific pathogens.

2. An antigen (antibody-generator) is a molecule that induces an adaptive immune response. An antibody is an immune protein (immunoglobulin) found in blood plasma that attaches to one particular kind of antigen and helps counter its effects.

3. Adaptive immunity has a remarkable memory; it can remember antigens it has encountered before.

4. Adaptive immunity is usually obtained by natural exposure to antigens, but it can also be achieved by vaccination, also known as immunization. A vaccine composed of a harmless variant or part of a disease-causing microbe, a dead or weakened virus.

5. Active immunity is any immunity an organism creates for itself as a response to a specific antigen naturally (if you catch the flu) or artificially (if you get a flu vaccine). Passive immunity is a temporary immunity that an organism receives the active premade antibodies.

6. The lymphatic system is involved in both innate and adaptive immunity, including a branching network of vessels, lymph nodes, bone marrow, and several organs.

(1)  Lymph nodes contain masses of lymphocytes and macrophages.

(2)  Lymphocytes are the white blood cells responsible for adaptive immunity. They are found in the blood and also in the tissues and organs of the lymphatic system.

(3)  The lymphatic vessels carry the lymph fluid, which is similar to the interstitial fluid in a body.

(4)  The interstitial fluid from the blood in a capillary bed enters the lymphatic system by diffusing into dead-end lymphatic capillaries.

7. Lymphocytes originates from two places:

(1)  Bursa of fabricius in birds or bone marrow in mammals: B lymphocytes (B cells) secrete antibodies responding in the humoral immune response

(2)  Thymus: T lymphocytes (T cells) responding in the cell-mediated immune response

8. Antigenic determinant (epitope) is a small surface-exposed region of an antigen molecule. Antigen-binding site is the specific region on the antibody that recognizes and binds an antigenic determinant. Both antigen-binding site and antigenic determinant have special shapes like a lock and key.

9. Clonal selection theory is the antigen-driven cloning of lymphocytes. This theory states that an antigen can activates (selects) its counter-specific B and T cells from a pre-existing pool of lymphocytes in an organism. Once activated (selected) by the antigen, the lymphocytes proliferate, forming a clone (a genetically identical population) of thousands of cells to recognize and response to the antigen.

(1)  Effector cells: some of these activated (selected) lymphocytes act immediately to combat infection.

(2)  Memory cells: some of these activated (selected) lymphocytes lie in wait to help activate the immune system upon subsequent exposure to the same antigen.

Thus, according to the clonal selection theory, the adaptive immune system functions on the “ready-made” rather than the “made-to-measure” principle.

10. The primary immune response is the initial adaptive immune response to an antigen, which appears to be slow after a lag of about 10 days. The secondary immune response is the adaptive immune response generated when an organism encounters the same antigen at some later time. Due to the production of memory cells, the secondary immune response is more rapid, of greater magnitude, and of longer duration than the primary immune response.

11. Antibodies do not kill pathogens. Instead, antibodies mark a pathogen by combining with it to form an antigen-antibody complex. Once marked in this manner, other (cell-mediated) immune system bring about the destruction of the antigen.

(1)  The shape of an antibody resembles a Y.

(2)  Each antibody molecule consists of 4 polypeptides, 2 heavy chains and 2 light chains.

(3)  The tip of each arm of antibody forms an antigen-binding site.

(4)  For example, moboclonal antibodies are used to detect pregnancy against human chorionic gonadotropin (hCG), which is present in the urine of pregnant women.

12. Herd immunity (community protection) works effectively to prevent the outbreak of infectious disease, if the level of immunization to a specific pathogen is maintained at 95% or higher.

13. Helper T (T_H) cells triggers both the humoral and cell-mediated immune responses. Signals from T_H cells activate B cells to initiate production of antibodies (plasma cells) that neutralize pathogens, and activate cytotoxic T (T_C) cells that kill infected cells.

14. The role of helper T cells is so important to adaptive immunity that without functional helper T cells, there is no immune response. Two requirements must be met for a helper T cell to active adaptive immune responses:

(1)  A foreign molecule must be present that can bind specifically to the T-cell receptor (TCR) of the T cell.

(2)  This foreign antigen must be displayed by major histocompatibility complex (MHC) molecules on the surface of an antigen-presenting cells (APCs). Macrophages and B cells are two types of antigen-presenting cells.

15. The cytotoxic T cells are the only T cells that actually kill infected cells.

(1)  A cytotoxic T cell binds to the infected cell, and synthesizes several toxic proteins, for instance, perforin and granzyme.

(2)  Perforin makes holes in the infected cell.

(3)  T cell enzymes (granzyme) then enter the infected cell and promote its death by apoptosis (programmed cell death).

16. The human immunodeficiency virus (HIV) attacks and destroys helper T (T_H) cells in humans with acquired immunodeficiency syndrome (AIDS).

III. Disorders of the Immune System

1. Autoimmune disorder is an immunological disorder in which the immune system attacks the bodyꞌs own molecules.

(1)  In systemic lupus erythematosus (SLE), B cells produce antibodies against a wide range of self molecules, such as histones and DNA.

(2)  In rheumatoid arthritis, antibodies attacks synovium, a thin layer of joints.

2. Immunodeficiency disorder is an immunological disorder in which the immune system lacks one or more components, making the body susceptible to infectious agents that would ordinarily not be pathogenic.

(1)  In severe combined immunodeficiency (SCID), both T and B cells are absent or inactive.

(2)  In Hodgkinꞌs disease, a type of cancer that damages the lymphocytic system and can depress the immune system.

3. Allergies are overreactions to certain environmental antigens. Allergies are hypersensitive responses to harmless antigens (allergens) in our surroundings.

(1)  Common allergens include protein molecules on pollen grains, on the feces of tiny mites, and in the animalsꞌ saliva on the fur.

(2)  Symptoms of an allergy result from a two-stage reaction:

1)    Sensitization: a person is exposed to an allergen, and B cells proliferate through clonal selection and secrete large amounts of antibodies to this allergen.

2)    Exposure: the person is exposed to the same allergen. This allergen enters the body and binds to the antibodies attached to mast cells. This causes the mast cells to release histamine, which causes blood vessels to dilate and leak fluid, leading to nasal irritation, itchy skin, and tears. Antihistamines are drugs that interfere with histamineꞌs action and give temporary relief from an allergy.

(3)  Allergies range from seasonal diseases to severe, life-threatening responses. Anaphylactic shock is a dangerous allergic reaction for people sensitive to bee venom, penicillin, peanuts, or shellfish. Anaphylactic shock can be counteracted with epinephrine.