Introduction to Immunology

Module Objectives

• Enables students to understand basic immunology concepts.
• Facilitates understanding of how the immune system functions and responds.

Semester Outcomes

• Students should explain immunologic concepts and their links.
• Students should correlate immunologic concepts in clinical and environmental scenarios.
• Students should use correct immunology terminology for effective communication.
• Students should use appropriate methods to analyze and present immunologic problems with precision and safety.

References

• Decker, Janet M. (2000). Introduction to Immunology. Blackwell Science Company.
• Wise, Darla J., and Gordon R. Carter (2002). Immunology: A Comprehensive Review. Blackwell Science Company.
• Roitt, Ivan M., and Peter J. Delves (2001). Roitt’s Essential Immunology. Blackwell Publishing.

Problem Examples

• Why alcoholics are more susceptible to bacterial pneumonia than the general population.
• How to create a biological weapon to cause deadly disease, including choice of molecule and why.

Properties of Immunity

• Immunity refers to the mechanisms the body uses to protect against foreign environmental agents.
• Environmental agents include microorganisms, foods, chemicals, drugs, pollen, animal hair, and dander.
• Immunity is either innate or acquired.

Innate Immunity

• Innate immunity is a mechanism linking cellular and subcellular components present from birth.
• It involves anatomical, cellular, and humoral defenses that function early in response to foreign substances.
• Innate immunity blocks/limits access or activates mechanisms to eliminate foreign particles.

Elements of Innate Immunity

• Cells: phagocytes, basophils, eosinophils, natural killer cells, antigen-presenting cells.
• Humoral factors: complement proteins, lysozymes, pepsin, stomach acidity, lactoferrin.
• Anatomical barriers: cilia, mucus, skin.
• Resident flora: nonpathogenic bacteria in the nasopharynx, intestine, and vagina.
• Pattern recognition molecules: Toll-like receptors.

Characteristics of Innate Immunity

• Always present and quickly available.
• Cells can only distinguish between self and non-self, so it's non-specific.
• Indirectly activates acquired immunity by secreting proteins that influence its course.

Acquired Immunity

• Acquired throughout life.
• Also known as adaptive immunity.
• Only present in vertebrates.
• Individuals are born with the ability to respond to foreign substances, but B and T cell numbers are initially inadequate.
• B and T cell numbers increase after contact with an antigen through activation of lymphocytes with antigen-specific receptors.

Elements of Acquired Immunity

• Cells: lymphocytes.
• Humoral factors: antibodies and cytokines.
• All elements work together to defend the host body.
• Each element can work alone or prepare the foreign substance for destruction by other cells.

Lymphocytes

• Consist of B cells and T cells.
• B cells are also known as membrane antibodies or membrane immunoglobulin.
• Activated B cells differentiate into memory B cells or plasma cells.
• Two types of T cells: helper T cells and cytotoxic T cells.
• Helper T cells secrete cytokines.
• Cytotoxic T cells destroy virally infected host cells.

Antigenic Stimulation

• Antigenic stimulation of B cells, T cells, and antigen-presenting cells initiates a chain of events leading to cell proliferation.
• This process takes days to weeks; innate immunity plays an important role as the first line of defense.
• Acquired immunity has a memory response due to its specificity.

Antigen and Immune Recognition

• Antigen: a foreign substance that triggers the immune system.
• Antigenicity or immunogenicity describes the effectiveness of molecules in activating adaptive immunity.
• Only a portion of a large antigen molecule is used for recognition (the "nametag").
• The specific region used to identify an antigen is known as antigenic determinants or epitopes.

Properties of Immunogenic Substances

• Common properties include being foreign, chemically complex, and having a high molecular weight.
• Foreign molecules can be proteins expressed by mutated cells (tumor-specific antigens).
• Molecules synthesized in a laboratory can be antigens (synthetic or recombinant proteins).

Chemical Complexity and Immunogenicity

• Chemically complex means comprising many different building blocks.
• Polysaccharides have repeating units: a weak immunogen, even with foreignness and high molecular weight.
• Proteins have around 20 building blocks, making them more immunogenic than polysaccharides.

Molecular Weight and Immunogenicity

• The most potent antigens usually have high molecular weights.
• Molecules under 1 kilodalton are considered nonimmunogenic.
• Molecules between 1 kD and 6 kD may be weakly immunogenic.
• Small molecules (under 1 kD) that are potentially immunogenic are called haptens.

Haptens

• When combined with a larger protein, a hapten creates a hapten-carrier complex, potentially highly immunogenic.
• A high molecular weight molecule coupled with potentially immunogenic substance cannot be identified as a hapten.
• Criteria for a hapten: Potentially immunogenic when coupled with other molecule and has low molecular weight.

Relative Immunogenicity

• Immunogenicity varies with foreignness, chemical complexity, and molecular size.
• Proteins: ++++
• Polysaccharides: ++
• Lipids: +
• Nucleic Acids: +.

Antigen Recognition Specificity

• Innate immunity is non-specific, while acquired immunity is specific.
• Innate immunity distinguishes between self and non-self.
• Acquired immunity distinguishes between self and non-self, bacteria, virally infected cells, etc.

Antigen Recognition by Innate Immunity

• Two approaches: direct and indirect.
• The main difference is the use of a binding molecule called opsonin.

Antigen Recognition by Acquired Immunity

• B cells and T cells are the main components.
• These cells have antigen-recognizing receptors that bind to the antigen epitope.
• B cells recognize antigens via receptors on their cell surface (membrane immunoglobulin).
• B cells interact with intact antigens.

T Cell Recognition

• T cells recognize antigens via T cell receptors.
• T cell receptors only recognize antigenic peptides + MHC protein.
• Antigen-presenting cells degrade antigens into small peptides (antigenic peptides).
• Antigenic peptides + major histocompatibility complex (MHC) = visible to T cell receptors.

Summary of Immunity

• Innate immunity: the first line of defense.
• Acquired immunity is activated if a foreign substance enters the body.
• Innate immunity includes anatomical barriers, resident flora, humoral factors, and cells.
• Acquired immunity involves cells and humoral factors.
• There is antigen recognition in both innate and acquired immunity.