Basic Immunology

Fadel Muhammad Garishah

Immune System

The cells and molecules responsible for immunity constitute the immune system, and their collective and coordinated response to the introduction of foreign substances is called the immune response.

Innate Immunity• Innate immunity (also called natural or native immunity) consists of

cellular and biochemical defense mechanisms that are in place.• The principal components of innate immunity are

– (1) physical and chemical barriers, such as epithelia and antimicrobial substances produced at epithelial surfaces;

– (2) phagocytic cells (neutrophils, macrophages) and NK (natural killer) cells;

– (3) blood proteins, including members of the complement system and other mediators of inflammation; and

– (4) proteins called cytokines that regulate and coordinate many of the activities of the cells of innate immunity.

• Innate immunity provides the early lines of defense against microbes and may not distinguish specifically differences between foreign substances.

Adaptive Immunity

• Because this form of immunity develops as a response to infection and adapts to the infection, it is called adaptive immunity.

• Specific immunity.• Cellular Immunity (Macrophages, Antigen

Presenting Cells, T lymphocytes)• Humoral immunity (B lymphocytes,

Plasmocytes/Plasma Cells and Immunoglobulins)

Development of Lymphocytes

Hematopoiesis

IL-5

Innate ImmunityEpithelial Barier

Epithelia produce peptides that have a natural antibiotic function- Defensins- intraepithelial T

lymphocytes and the B-1 subset of B cells, respectively, and these cells may recognize and respond to commonly encountered microbes.

Innate ImmunityPhagocytes

Neutrophils and monocytes are recruited from the blood to sites of infection by binding to adhesion molecules on endothelial cells and by chemoattractants produced in response to the infection.

Innate ImmunityNatural Killer Cells

A. NK cells kill host cells infected by intracellular microbes, thus eliminating reservoirs of infection.

B. NK cells respond to IL-12 produced by macrophages and secrete IFN-γ, which activates the macrophages to kill phagocytosed microbes.

NK cell activation is regulated by a balance between signals that are generated from activating receptors and inhibitory receptors

Innate ImmunityComplement System

The complement system consists of several plasma proteins that are activated by microbes and promote destruction of the microbes and inflammation.

Other Circulating Effector Proteins of Innate Immunity

• Mannose-binding lectin, is a plasma protein that functions as an opsonin.

• C-reactive protein is a plasma protein that typically binds to bacterial phospholipids. It functions as an opsonin.

• C-reactive protein is called an acute-phase reactant because its plasma levels increase during the acute stages of many infections.

• Coagulation factors are plasma proteins that mainly function to prevent hemorrhage by forming a thrombus at sites where blood vessel integrity is broken.

Cytokines of Innate Immunity

• The cytokines of innate immunity recruit and activate leukocytes and produce systemic alterations, including increases in the synthesis of effector cells and proteins that potentiate antimicrobial responses.

Role of Innate Immunity in Stimulating Adaptive Immune Responses

A. Macrophages and dendritic cells respond to phagocytosed (cell-associated) microbes, to activate T lymphocytes

B. B lymphocytes recognize microbial antigens by their antigen receptors and recognize complement system, to activate the B cells.

Adaptive Immunity

Cellular Immunity is mediated by T lymphocytes (also called T cells).

Humoral immunity is mediated by molecules in the blood and mucosal secretions, called antibodies, that are produced by cells called B lymphocytes

Active and passive immunity.

Adaptive Immune Response

• Specificity Ensures that distinct antigens elicit specific responses

• Diversity Enables immune system to respond to a large variety of antigens

• Memory Leads to enhanced responses to repeated exposures to the same antigens

• Specialization Generates responses that are optimal for defense against different types of microbes

• Self-limitation Allows immune system to respond to newly encountered antigens

• Nonreactivity to self Prevents injury to the host during responses to foreign antigens

The principal cells of the immune system are lymphocytes, antigen-presenting cells, and effector cells.

Simplified illustration

Mucosal Immunity

Skin Immunity