Immune System Chapter 14

Immunity – the ability to resist infection and disease Humans have two major types of defense mechanisms: Innate (Non-specific) immunity Adaptive (Specific) immunity

Innate (Non-specific) immunity defense responses that do not distinguish between one threat and another are present at birth include: physical barriers (e.g. skin), phagocytic cells (neutrophils, monocytes, macrophages, eosinophils), chemicals (complement system), inflammation, fevers, etc. provides body with “non-specific resistance”

Adaptive (Specific) immunity protection 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 usually produce a state of long-term protection known as “specific resistance” or “acquired immunity”

Adaptive (Specific) 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)

Adaptive (Specific) immunity arises throughout life by active or passive means

Active immunity – development of resistance (i. e 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 – antibodies transferred from mom  baby across placenta or in breast-milk induced (artificial) passive immunity – administration of antibodies to fight disease after exposure to pathogen

Properties of Specific (Adaptive) Immunity Immunity has four general properties: Specificity Versatility Memory Tolerance

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 (aka CD8 cells) - seek out the specific pathogen/infected cell that contains the targeted Ag & destroys it by secreting various chemicals helper T cells (aka CD4 cells) – necessary for coordination of specific (adaptive) & non-specific (innate) 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 cells Helper T cells Memory T cells Suppressor T cells Direct physical & chemical attack stimulate B cell activation ANTIGENS bacteria viruses SPECIFIC DEFENSES (Immune response) CELL MEDIATED IMMUNITY APC’s phagocytize Ag & activate T cells Prevent autoimmune response Antigens Remain in reserve bacteria viruses

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

Summary of Defense and Immune Responses