Immune System

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The Immune System
CHAPTER 4

Learning objectives
• By the end of this chapter, students should be able to : i) Differentiate the three line of defense system in the human body ii) Describe the agglutination process in the case of the ABO blood group iii) Differentiate between active and passive immunization iv) Discuss the homeostatic imbalances in the immune system

Introduction
• Barriers help an animal to defend itself from the many dangerous pathogens it may encounter

• The immune system recognizes foreign bodies and responds with the production of immune cells and proteins • Two major kinds of defense have evolved: a) innate immunity b) acquired immunity

Innate immunity
• Also known as external defenses • It involves nonspecific responses to pathogens • Nonspecific defense mechanism can be divided into two lines. • The first lines of defenses are barriers at body surfaces. They help fight undistinguishable pathogens that enter our body.

Barriers at body surface
1) Skin
Oil and sweat gland secretions acidify the skin. pH at about 3.5-5 (discourage bacterial growth) The skin itself is a tough and intact barrier of dead skin cells that most bacteria and viruses cannot penetrate.

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2) Protective fluids - The body secretes protective fluid tha trap foreign agents - Lysozyme - enzyme that attacks the cell wall of many bacteria - The body also excretes several body fluids such as saliva, tears and perspiration containing protective substance

3) Defenses of the respiratory system - The hair in our nostrils filters the incoming air. - Mucous membrane - traps most microbes and dirt that get past the nasal filter. - Cillia on cells lining the tubes sweep the mucous upward and downward out of the system.

4) Defenses of the digestive system • Stomach acids kill many bacteria that enter the stomach with food, • Lysozyme in saliva

5) Defenses of the urogenital tract • Low pH - kills bacteria from the urinary tract. This is accompanied by the flushing action that washes away pathogens. • Vagina secretions - sticky and acidic. - killing invading bacteria

• Second line of defense – cellular counterattack/cellular innate defenses • These cells respond to ANY microbial infection without needing to determine the invader‟s identity. • Can be divided into : a) phagocytosis, b) actions by antimicrobial proteins, c) natural killer cells d) inflammatory responses Nonspecific defensive cells

Phacocytosis • White blood cells –they are phagocytes
a) Neutrophils - engulf and destroy microbes b) Macrophages - part of the lymphatic system and found throughout the body. They roam continuously in the extracellular fluid and engulf viruses, cellular debris and also dust particles at the lungs.

phagocytosis,-Nonspecific defensive cells

c) Eosinophils - defend the body against multicellular parasitic invaders such as blood fluke by discharging destructive enzymes d) Dendritic cells – ingest microbes and also play a role as an antigen-representing cell.

phagocytosis,-Nonspecific defensive cells

• Phagocytes will attach to foreign agents that enter the body by recognizing certain structures eg polysaccharides on the surface of bacteria • After engulfing them, vacuole will be formed that will fuse with a lysosome i. Nitric oxide can poison the engulfed microbes ii. Lysozyme degrade microbial components phagocytosis,-Nonspecific defensive cells

b) Antimicrobial proteins • Interferon is proteins produced by virus-infected cells that help other cells in the immediate vicinity to resist viral infection by the production of substances inhibiting viral reproduction. • Limits cell to cell spread of virus – control viral infections such as colds and influenza • Complement proteins are proteins that circulate in an inactive forming the blood plasma. • They attack microbes by coating the surfaces of microbes in order to make it easier for the macrophages to engulf and also by making lethal...
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