Chapter 43 ~ The Body’s Defenses
Lines of Defense
Phagocytic and Natural Killer Cells
Neutrophils 60-70% WBCs; engulf and destroy microbes at infected tissue
Monocytes 5% WBCs; develop into….
Macrophages enzymatically destroy microbes
Eosinophils 1.5% WBCs; destroy large parasitic invaders (blood flukes)
Natural killer (NK) cells destroy virus-infected body cells & abnormal cells
The Inflammatory Response
1- Tissue injury; release of chemical signals~ • histamine (basophils/mast cells): causes Step 2... • prostaglandins: increases blood flow & vessel permeability
2/3- Dilation and increased permeability of capillary~ • chemokines: secreted by blood vessel endothelial cells mediates phagocytotic migration of WBCs
4- Phagocytosis of pathogens~ • fever & pyrogens: leukocyte-released molecules increase body temperature
Specific Immunity
Lymphocyctes •pluripotent stem cells... • B Cells (bone marrow) • T Cells (thymus)
Antigen: a foreign molecule that elicits a response by lymphocytes (virus, bacteria, fungus, protozoa, parasitic worms)
Antibodies: antigen-binding immunoglobulin, produced by B cells
Antigen receptors: plasma membrane receptors on b and T cells
Clonal selection
Effector cells: short-lived cells that combat the antigen
Memory cells: long-lived cells that bear receptors for the antigen
Clonal selection: antigen-driven cloning of lymphocytes
“Each antigen, by binding to specific receptors, selectively activates a tiny fraction of cells from the body’s diverse pool of lymphocytes; this relatively small number of selected cells gives rise to clones of thousands of cells, all specific for and dedicated to eliminating the antigen.”
Induction of Immune Responses
Primary immune response: lymphocyte proliferation and differentiation the 1st time the body is exposed to an antigen
Plasma cells: antibody-producing effector B-cells
Secondary immune response: immune response if the individual is exposed to the same antigen at some later time ~ Immunological memory
Self/Nonself Recognition
Self-tolerance: capacity to distinguish self from non-self
Autoimmune diseases: failure of self-tolerance; multiple sclerosis, lupus, rheumatoid arthritis, insulin-dependent diabetes mellitus
Major Histocompatability Complex (MHC): body cell surface antigens coded by a family of genes
Class I MHC molecules: found on all nucleated cells
Class II MHC molecules: found on macrophages, B cells, and activated T cells
Antigen presentation: process by which an MHC molecule “presents’ an intracellular protein to an antigen receptor on a nearby T cell
Cytotoxic T cells (T C ): bind to protein fragments displayed on class I MHC molecules
Helper T cells (T H ): bind to proteins displayed by class II MHC molecules
Types of immune responses
Humoral immunity
B cell activation
Production of antibodies
Defend against bacteria, toxins, and viruses free in the lymph and blood plasma
Cell-mediated immunity
T cell activation
Binds to and/or lyses cells
Defend against cells infected with bacteria, viruses, fungi, protozoa, and parasites; nonself interaction
Helper T lymphocytes
Function in both humoral & cell-mediated immunity
Stimulated by antigen presenting cells (APCs)
T cell surface protein CD4 enhances activation
Cytokines secreted (stimulate other lymphocytes): a) interleukin-2 (IL-2): activates B cells and cytotoxic T cells b) interleukin-1 (IL-1): activates helper T cell to produce IL-2
Helper T Cell Activity
Cell-mediated: cytotoxic T cells
Destroy cells infected by intracellular pathogens and cancer cells
Class I MHC molecules (nucleated body cells) expose foreign proteins
Activity enhanced by CD8 surface protein present on most cytotoxic T cells (similar to CD4 and class II MHC)
T C cell releases perforin, a protein that forms pores in the target cell membrane; cell lysis and pathogen exposure to circulating antibodies
Cytotoxic T Cell Activity
Humoral response: B cells
Stimulated by T-dependent antigens (help from TH cells)
Macrophage (APCs) with class II MHC proteins
Helper T cell (CD4 protein)
Activated T cell secretes IL-2 (cytokines) that activate B cell
B cell differentiates into memory and plasma cells (antibodies)
Humoral Response
Antibody Structure & Function
Epitope: region on antigen surface recognized by antibodies
2 heavy chains and 2 light chains joined by disulfide bridges
Antigen-binding site (variable region)
5 classes of Immunoglobins
IgM : 1st to circulate; indicates infection; too large to cross placenta
IgG : most abundant; crosses walls of blood vessels and placenta; protects against bacteria, viruses, & toxins; activates complement
IgA : produced by cells in mucous membranes; prevent attachment of viruses/bacteria to epithelial surfaces; also found in saliva, tears, and perspiration
IgD : do not activate complement and cannot cross placenta; found on surfaces of B cells; probably help differentiation of B cells into plasma and memory cells
IgE : very large; small quantity; releases histamines-allergic reaction
Antibody-mediated Antigen Disposal
Neutralization (opsonization): antibody binds to and blocks antigen activity
Agglutination: antigen clumping
Precipitation: cross-linking of soluble antigens
Complement fixation: activation of 20 serum proteins, through cascading action, lyse viruses and pathogenic cells
Antibody Action
Immunity in Health & Disease
Active immunity/natural: conferred immunity by recovering from disease
Active immunity/artificial: immunization and vaccination; produces a primary response
Passive immunity: transfer of immunity from one individual to another • natural: mother to fetus; breast milk • artificial: rabies antibodies
ABO blood groups (antigen presence)
Rh factor (blood cell antigen); Rh- mother vs. an Rh+ fetus (inherited from father)
Abnormal immune function
Allergies (anaphylactic shock): hypersensitive responses to environmental antigens (allergens); causes dilation and blood vessel permeability (antihistamines); epinephrine
Autoimmune disease: multiple sclerosis, lupus, rheumatoid arthritis, insulin-dependent diabetes mellitus
Immunodeficiency disease: SCIDS (bubble-boy); A.I.D.S.
