Immune System Chapter 14 Humans have two major

Immune System Chapter 14

Humans have two major types of defense mechanisms: Non-specific defenses & Specific defenses § Non-specific defenses § do not distinguish between one threat and another § are present at birth § include: physical barriers (e. g. skin), phagocytic cells, inflammation, fevers, etc. § provides body with “non-specific resistance”

§ Specific defenses § protect against specifically identified threats (i. e. may defend against one particular bacterial infection but not a different one) § most develop after birth upon exposure to an antigen (Ag); an antigen can be a pathogen (disease-causing organism), foreign protein (e. g. toxin), abnormal or infected body cell, foreign tissue transplant § specific defenses produce a state of long-term protection known as “specific resistance” or “Immunity”

Immunity § depends on coordinated activity of T & B lymphocytes § T cells- involved in “cell-mediated (aka cellular) immunity”; defense against abnormal cells & intracellular pathogens § B cells- involved in “antibody-mediated (aka humoral) immunity”; defense against pathogens (Ag’s) in body fluids (blood/lymph)

Overview of Immunity Fig. 14 -11)

Immunity is either “innate” or “acquired” Innate Immunity § present at birth § independent of previous exposure to Ag § genetically determined § species dependent

Acquired Immunity § arises throughout life by active or passive means

Active immunity – development of resistance (i. e. antibody (Ab) production) to specific disease secondary to exposure to specific Ag (pathogen) § naturally acquired active immunity – natural exposure results in immune response & development of long term immunity § induced (artificial) active immunity – deliberate “artificial” exposure to Ag (i. e. vaccine/immunization)

Passive immunity – development of immunity due to transfer of “pre-made” antibodies § naturally acquired passive immunity – Ab’s transferred from mom baby across placenta or in breast-milk § induced (artificial) passive immunity – administration of Ab’s to fight disease after exposure to pathogen

Properties of Immunity has four general properties: ¨ Specificity ¨ Versatility ¨ Memory ¨ Tolerance Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings

Properties of Immunity § Specificity – T & B cells have specific receptors that will allow them to only recognize & target a specific Ag; this process is known as “antigen recognition” § Versatility – millions of different lymphocyte populations, each with specific Ag recognizing receptors; allows for “anticipation” of potential Ag’s § Memory – after initial exposure, long term acquired immunity occurs through the production of memory cells; secondary exposure results in stronger faster response to previously recognized Ag § Tolerance – immune cells recognize self-antigens & “tolerate” (ignore) them, only going after foreign (non-self) Ag’s

Overview of the immune response § The purpose of the immune response is to inactivate or destroy pathogens, abnormal cells & foreign molecules (such as toxins) § In order for the response to occur, lymphocytes must be “activated” by the process of antigen recognition § T cells are usually activated first, & then B cells. T cells mainly rely on activation by phagocytic cells collectively known as “antigen presenting cells (APC’s)” (ie. Macrophages, dendritic cells) § Once activated, T cells both attack the invader, & stimulate the activation of B cells § Activated B cells mature into “plasma cells” which produce specific antibodies designed to destroy the particular antigen.

Cell Mediated (a. k. a. Cellular) Immunity § In order for T cells to respond, they must first be activated by exposure to an antigen § antigen is bound to membrane receptors of phagocytic antigen presenting cells (APC’s) (“antigen recognition”) § These membrane receptors on cells are called “MHC proteins” (major histocompatibility complex proteins), & are genetically determined (i. e. differ among individuals) § Antigens bound to MHC proteins “tell” the T lymphocyte what the specific foreign invader is (i. e. a specific bacteria) so that the lymphocytes can mount a cellular defense

Cell Mediated (a. k. a. Cellular) Immunity Once a T cell is activated by the presentation of the combined MHC/Ag, it will clone (by mitosis) & differentiate into: § cytotoxic T cells § helper T cells § memory T cells § suppressor T cells

Cell Mediated (a. k. a. Cellular) Immunity § cytotoxic T cells – seek out the specific pathogen/infected cell that contains the targeted Ag & destroys it by secreting various chemicals § helper T cells – necessary for coordination of both specific & non-specific defenses, as well as for stimulating both cell-mediated & antibody-mediated immunity. § In cell-mediated immunity they release chemicals (cytokines) that strengthen the activity of cytotoxic T cells. § In antibody-mediated immunity they release cytokines that stimulate activated B cell division & differentiation into plasma cells

Cell Mediated (a. k. a. Cellular) Immunity § memory T cells – remain “in reserve” so if same Ag appears, these cells can immediately differentiate into cytotoxic & helper T cells, causing a swift secondary response to the invasion § suppressor T cells – activated more slowly than the other T cells; inhibit the response of the immune cells to prevent potential “autoimmune” response

Activated T cells clone & differentiate into: § Cytotoxic T cellsstimulate § Helper T cells § Memory T cells § Suppressor T cells Direct physical & chemical attack Antigens Remain in reserve B cell activation Prevent autoimmune response CELL MEDIATED IMMUNITY bacteria ANTIGENS viruses bacteria viruses SPECIFIC DEFENSES (Immune response) APC’s phagocytize Ag & activate T cells

Antibody Mediated (Humoral) Immunity §The body has millions of different B cell populations, each B cell has its own particular antibody (Ab) molecule (transmembrane protein) within its cell membrane § When the corresponding Ag invades the interstitial fluid surrounding the B cell, the Ag binds to the Ab molecule, & is taken into the cell, eventually being displayed on the B cell’s MHC protein. The B cell is now “sensitized”

Antibody Mediated (Humoral) Immunity § Helper T cells (that had been previously activated to the same Ag) then attach to the sensitized B cells & activate them by secreting chemicals (cytokines) § Cytokine secretion results in B cell cloning & differentiation into plasma cells & memory cells

Antibody Mediated (Humoral) Immunity § Plasma cells produce millions of copies of antibodies which are released into the blood & lymph § Antibodies seek out & bind to the Ag forming an “Ab-Ag complex”, eventually leading to the elimination of the antigen by various means § Memory cells remain in reserve to respond to any subsequent exposure by the same Ag. Upon secondary exposure, memory B cells quickly differentiate into Ab producing plasma cells

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Antibody Mediated (Humoral) Immunity

Review of Immune Response