Innate Immunity

General Characteristics
- Recognition of traits shared by broad ranges of pathogens using a small set of receptors
- Rapid response
Vs. Acquired Immunity
- Recognition of traits specific to particular pathogens using a vast array of receptors
- Slower response

Pathogens include microorganisms and viruses
Barrier Defenses
- Skin
- Mucous membranes
- Secretions (gastric juice, saliva, tears)
Internal Defenses
- Phagocytic cells
- Antimicrobial proteins
- Inflammatory response
- Natural killer cells
Responses
- Humoral response: Antibodies defend against infection in body fluids
- Cell-mediated response: Cytotoxic lymphocytes defend against infection in body cells

Defend self from non-self
Recognize self from non-self through molecular recognition
Species are under constant attack by pathogens
- Immediate response within about 2 weeks

Barrier Defenses
- Chitin exoskeleton (insects)
- Skin (pH = 3-5)
- Mucus and cilia
- Body secretions like gastric juice, saliva, and tears which contain lysozymes
Pathogen Recognition
- If barriers are breached, recognition of foreign cells occurs
- Pathogens are detected by leukocytes (white blood cells)

Function
- Recognize molecules commonly found in viruses, fungi, and bacteria (e.g., double-stranded RNA, proteins, polysaccharides)
Main Types
- Neutrophils
- Macrophages
- Natural killer cells
Mechanism
- Neutrophils and macrophages engulf and destroy microbes via phagocytosis
  - Engulfed microbes are wrapped in a membrane and exposed to hydrolytic enzymes

Role
- Circulate through body fluids to detect abnormal surface proteins on cancerous or virus-infected cells
- Release chemicals that trigger apoptosis of affected cells
Signals
- Neutrophils attracted to signals from infected tissues
- Macrophages patrol and circulate to find pathogens and non-self proteins

Function
- Produced by leukocytes and virus-infected cells
- Interfere with viral infection and help activate macrophages
Mechanism
- Virus-infected cells secrete interferons to induce nearby cells to produce substances that inhibit viral replication, helping to limit the spread of infection
- Recombinant DNA technology is now used to produce interferons for viral infections treatment (e.g., hepatitis C)

Components
- Approximately 30 different blood plasma proteins (complement proteins) activated by contact with substances on the surface of many microbes
Functions
- Can lead to lysis of microbial cells
- Part of both innate and acquired immunity

Activation
- Triggered by histamine (a signaling molecule from mast cells)
- Causes dilation of blood vessels and increased permeability
- Results in localized swelling and attracts neutrophils and macrophages
Cytokines
- Released by activated leukocytes to promote blood flow and deliver complement proteins
- Results in pus formation (dead pathogens, leukocytes, cell debris)
Systemic Response
- Triggered by injured cells or infected tissues which stimulate leukocyte production in bone marrow and can cause fever

Septic Shock
- Overwhelming inflammatory response due to certain bacterial infections
- Characterized by high fever, low blood pressure, low blood flow through capillaries
- Fatal in ~1/3 of cases (approximately 200,000 in the U.S. annually)
Chronic Conditions
- Examples include Crohn’s disease, ulcerative colitis (autoimmune disorders affecting the intestines) and rheumatoid arthritis (causing swelling in joints due to increased blood flow triggered by histamines)

Key Components
- Leukocytes (neutrophils and macrophages) perform phagocytosis; natural killer cells induce apoptosis
Recognition Mechanisms
- Cell receptors and circulating complement proteins recognize common molecules of pathogens (viruses, fungi, bacteria) but not of animals
Interferons & Histamines
- Produced by leukocytes and infected cells to combat infections and increase blood flow, promoting attraction and activation of leukocytes who further activate complement proteins.