Skin and mucous membranes - first line of defense - represent barriers to infection - secretions may contain antimicrobial proteins (e.g., lysozyme)

Phagocytic cells, antimicrobial proteins and inflammatory response - second line of defense - nonspecific - affect wide range of pathogens

Neutrophils - constitute 60-70% of all white blood cells - attracted by chemical signals - life span short - tend to self-destruct after phagocytosis

Monocytes - circulate for a few hours, then develop into large macrophages ("big eaters) - enter tissues

Eosinophils - defense against larger invaders - e.g., parasitic worms

Natural Killer (NK) Cells - do not attack microorganisms directly - attack membrane of virus-infected or abnormal cells - cause lysis

Lysozyme - present in tears, saliva & nucous secretions - digests cell wall of certain bacteria

Complement system - complex of 20 serum proteins - reactions that cause lysis of microbes - some involved in chemotaxis

Interferons - antiviral proteins secreted by virus-infected cells - induce nearby cells to produce chemicals that impede viral reproduction - not specific

Body’s third line of defense

Provides highly-specific defense(s) against foreign molecules (antigens)

Components provide immediate defense against invaders and confer long-term immunity to certain pathogens

Involves activities of several types of lymphocytes (white blood cells)

Foreign molecule (e.g., polypeptide of virus) or abnormal (e.g., cancerous) cell

Distinctive configuration of antigen molecule (epitope) recognized as "nonself" by specific receptors of lymphocyte(s)

A given pathogen or abnormal cell may provide numerous antigens

B Cells - develop in bone marrow - when activated, secrete antibodies - humoral response

Cytotoxic T Cells - develop in thymus -when activated, attacks and destroys infected body cells - cell-mediated response

Helper T Cells - responds to antigen - when activated, mediates both branches of immune system

Develop in bone marrow

May ingest antigens through receptor-mediated endocytosis

When activated,secrete antibodies with receptors specific to particular antigen - causes variety of effects

Antibody - antigen binding immunoglobulin - secreted by B cells - receptors specific for particular epitope of antigen

Binds to epitope(s) of antigen - may impede infection of cell, render antigen more susceptible to phagocytosis (opsonization), cause antigens to agglutinate or precipitate, or activate complement protein system

Develop in thymus

When activated, become active killers - destroy infected or abnormal cells by discharge of protein (perforin) - allows water and ions to enter cell - causes cell to lyse

Effective defense against intracellular parasites and abnormal cells

Activated by exposure to antigen and cytokines (interleukin-1)

When activated, - secretes interleukin-2- activates both B cells (humoral response) and Cytotoxic T cells (cell-mediated response)

While developing in bone marrow (B cells) or thymus (T cells), receptors tested for potential self-reactivity - receptors reactive against native molecules rendered nonfunctional or destroyed by programmed cell death (apoptosis)

Receptors interact with major histocompatability complex (MHC) molecules - cell surface markers involved in antigen presentation

Failure of self-tolerance results in autoimmune disorders

Family of genes that code for group of cell surface glycoproteins (proteins w/ attached sugar chains - two major groups

Class I MHC - found in all nucleated cells

Class II MHC - occur in macrophages, B cells, activated T cells

Cell surface markers - important in antigen presentation and T cell activation

MHC molecule cradles fragment of antigen and presents it to T cell

Each antigen-MHC complex forms unique complex that is recognized by specific antigen receptors on certain T cells

Receptors of Cytotoxic T cells bind to Class I MHC molecules - receptors of Helper T cells bind to Class II MHC

Initial exposure to antigen "selects" lymphocyte with appropriate receptor

Selected B or T cell proliferates - produces clones of effector cells (for immediate defense) and memory cells (for long-term immunity) - "clonal selection" - clones highly specific for that particular antigen

Humoral (antibody-mediated) response - involves secretion of antibodies by activated B cells - marks foreign molecules or abnormal cells for destruction

Cell-mediated response - activated Cytotoxic T cells become active killers - destroy infected or abnormal cells

Both branches mediated by Helper T cells

Primary immune response - occurs following initial exposure to particular antigen - generates clones of effector cells and memory cells - specific to that particular antigen

Secondary immune response - occurs following subsequent exposure to antigen - response more pronounced - longer duration

Organism innoculated with dead or weakened microbes - unable to cause disease - acts as antigen - triggers immune response (humoral and cell-mediated)

Memory cells & antibodies confer long-term immunity to pathogen

Active - results from response(s) of organisms own immune system

Passive - results from transfer of antibodies from one individual to another - e.g., transfer of antibodies from mother to fetus through placenta - no memory cells

Allergies - hypersensitive responses to certain antigens - in extreme cases, may lead to anaphylactic shock

Autoimmune diseases - system loses self-tolerance - immune system attacks organism’s own cells - example: multiple sclerosis in which immune systems destroys myelin sheath in CNS

Immunodeficiency diseases - humoral and/or cell-mediated response adversely affected and/or disfunctional - HIV and AIDS