Introduction to Immunology

• Basics of Immunology

Learning Objectives

• The lecture introduces the science of immunology and its relevance to medical practice.
• Natural innate immunity is discussed.
• Natural infection barriers are presented.
• Mechanisms used the immune system to defend against agents that cause the body damage.

Contents Covered

• The primary topics include the immune system, natural and adaptive immunity, the natural barriers of infection, and the mechanisms of natural immunity.
• Also covers clinical cases, discussions, and assignments.

What is Immunity?

• Immunity refers to being protected and unaffected by harmful agents.
• Immunity aids the body in shielding itself from both external and internal threats.
• Immunology studies immune cells, mechanisms, reactions, and factors impacting the immune system, and the immunological causes of human diseases.

Case Scenario

• Coccobacillus tries to invade Gedd Ali's body by gathering numerous relatives and friends for the attack.
• The invaders face the barrier of healthy skin and manage to enter through openings like the mouth and nose.
• Invaders are attacked by lysozyme in the mouth and burned by hydrochloric acid(HCL) in the stomach, though some manage to escape.
• Some cross into the blood but are trapped by receptors (LTR).
• Phagocytes engulf some of these invaders.
• Other defenders armed with cytokines, complement, and C-reactive proteins are found in blood.
• Another group tries to access through a wound but, encounter cells that release chemical weapons.

The Immune System

• It is a combination of cells (mostly WBCs) and tissues (spleen and lymph nodes).
• It works to resists infection and harmful effects from other living things.
• Host immune mechanisms help identify pathogens and neutralize threats.
• The immune system can detect and identify disease-causing agents like viruses, bacteria, and parasites.
• A healthy immune system recognizes the difference between its healthy cells/tissues and foreign/damaged cells.

Immune System Functions

• Fights pathogens like bacteria, viruses, parasites, or fungi and eliminates them.
• Recognizes and neutralizes harmful environmental substances.
• Fights disease-causing changes in the body, such as cancer cells.

Types of Immunity

• Immunity can be classified into:
  • Innate (nonspecific) immunity
  • Adaptive (specific) immunity

Innate Immunity

• Innate immunity, or non-specific/in-born immunity, includes cells (leukocytes) and mechanisms (inflammation and phagocytosis).
• Present naturally in healthy individuals protecting them from infections.
• Physical natural barriers of infection are included.
• The innate immune system ignores the germ after it is eliminated and does not share information about it with the rest of the body.
• Thus, the body cannot prepare to fight the germ if reinfection occurs.

Components of Innate Immunity

• Physical natural barriers
• Phagocytes
• Toll-like Receptors (TLR)
• Cytokines
• Circulating proteins: Complement and C-reactive proteins

Adaptive Immunity

• Evolved later
• Pathogen specific
• Reacts only to organisms that induce response
• Requires time to mount response

Natural Barriers of Infection

• Include enzymes in mucus, tears, and saliva.
• Coughing and sneezing.
• Mucus and intact mucous membranes.
• Cilia in the respiratory tract that trap foreign material.
• Intact skin.
• Competition from commensal bacteria in the gut and genital tract.
• Acid in sweat and stomach.
• Anti-bacterial proteins and zinc in semen.

Phagocytes

• These cells of the immune system include macrophages, neutrophils, and dendritic cells.
• They engulf, or 'phagocytose', pathogens or particles.
• They react to mediators like cytokines and chemokines produced by other cells which attract and activate phagocytes.

Phagocytosis

• Phagocytes surround particle/pathogen by extending portions of their plasma membrane, wrapping the membrane until the particle/pathogen is inside the cell.
• Once inside, pathogens are contained within a phagosome.
• The phagosome merges with a lysosome, which contains enzymes and acids that kill and digest the particle or organism.
• Phagocytosis of host cells is part of tissue repair, development, and maintenance.
• Phagocytes remove dead host cells from the affected site.
• The process aids growth and development.

Inflammation

• The process of acute inflammation is initiated by cells already present in all tissues.
• Resident macrophages, dendrite cells, histiocytes, and mast cells.
• These cells release inflammatory mediators like histamine, bradykinin, serotonin, and prostaglandins due to an infection, burn, or other injuries.
• Inflammatory mediators sensitize pain receptors, cause local vasodilation of blood vessels, and attract phagocytes, especially neutrophils.
• Inflammation can cause pain, swelling, redness, hotness, and altered function.

Complement System

• Complements the ability of antibodies and phagocytic cells to clear microbes and damaged cells from the body.
• It is part of the innate immune system comprising components produced from the liver in an inactive form that is activated when needed.

Cytokines

• These (cyto, from the Greek word "cell" + kines, which mean "movement") are a group of small proteins or glycoproteins essential in cell signaling.
• The innate immune system has interleukins that work in communication between leukocytes.
• TNF (tumor necrosis factor) causes necrosis of malignant cells.
• IFN (interferon) interferes with viral infections.

Toll-Like Receptors (TLRs)

• TLRs are the first line of defense against microbes.
• There are 10 human subtypes of TLR.
• TLRs are expressed on innate immune cells like macrophages, neutrophils, dendritic cells (DCs), natural killer (NK) cells, mast cells, basophils, and eosinophils.
• External pathogen-associated molecular patterns (PAMPs) and internal damage-associated molecular patterns (DAMPs) are recognized by TLRs.
• TLR activation stimulates signaling cascades as a defense mechanism against invaders and repair damaged tissue.
• This leads to releasing various inflammatory cytokines and immune modulators.