Immunity

Introduction to Immunity

Immunity refers to the ability of an organism to resist infection or disease, primarily through the actions of the immune system. The immune system is divided into innate (nonspecific) and adaptive (specific) defenses. This guide focuses on the adaptive immune system, which provides targeted responses to specific pathogens.

The Adaptive Immune System: Third Line of Defense

Overview of Adaptive Immunity

• Adaptive immunity: Defenses that target a specific pathogen.

• Acquired through infection or vaccination.

• Primary response: The first time the immune system combats a particular foreign substance.

• Secondary response: Subsequent interactions with the same foreign substance; faster and more effective due to immunological "memory."

Dual Nature of the Adaptive Immune System

Humoral Immunity

Humoral immunity is mediated by antibodies produced by B lymphocytes. It is effective against pathogens and toxins found outside cells.

• Produces antibodies that combat foreign molecules known as antigens.

• B cells are lymphocytes created and matured in red bone marrow.

• B cells recognize antigens and produce antibodies specific to those antigens.

Cellular Immunity (Cell-Mediated Immunity)

Cellular immunity is mediated by T lymphocytes and is effective against pathogens that have entered host cells.

• Produces T lymphocytes that recognize antigenic peptides processed by phagocytic cells.

• T cells mature in the thymus.

• T cell receptors on the T cell surface contact antigens, causing the T cell to secrete cytokines instead of antibodies. Cytokines include interleukins, interferons, and others.

Comparison of Humoral and Cellular Immunity

• Cellular immunity attacks antigens that have already entered cells (e.g., viruses, some intracellular bacteria such as Mycobacterium leprae and Listeria monocytogenes).

• Humoral immunity fights invaders and threats outside cells (e.g., extracellular bacteria, toxins, and viruses before they enter host cells).

Cytokines: Chemical Messengers of Immune Cells

Functions of Major Cytokines

Cytokines are chemical messengers produced in response to a stimulus and regulate immune responses.

• Interleukins (ILs): Cytokines that mediate communication between leukocytes.

• Chemokines: Induce migration of leukocytes to sites of infection or inflammation.

• Interferons (IFNs): Interfere with viral infections of host cells.

• Tumor necrosis factor alpha (TNF-α): Involved in inflammation and autoimmune diseases.

• Hematopoietic cytokines: Control stem cells that develop into red and white blood cells.

• Overproduction of cytokines can lead to a cytokine storm, a potentially harmful excessive immune response.

Antigens

Definition and Properties

• Antigens: Substances that cause the production of antibodies; usually components of invading microbes or foreign substances.

• Haptens: Antigens too small to provoke immune responses; they must attach to carrier molecules to become immunogenic.

Humoral Immunity: Antibodies

Structure and Function of Antibodies

• Antibodies are globular proteins called immunoglobulins (Ig).

• Valence is the number of antigen-binding sites on an antibody molecule.

• Bivalent antibodies have two binding sites.

Major Classes of Immunoglobulins

• IgG: Monomer; 80% of serum antibodies; found in blood, lymph, and intestine. Crosses the placenta to protect the fetus, triggers complement, enhances phagocytosis, and neutralizes toxins and viruses.

• IgM: Pentamer made of five monomers held with a J chain; 6% of serum antibodies; remains in blood vessels; causes clumping of cells and viruses; first response to infection, short-lived.

• IgA: Monomer in serum, dimer in secretions; 13% of serum antibodies; common in mucous membranes, saliva, tears, and breast milk; prevents microbial attachment to mucous membranes.

• IgD: Monomer; 0.02% of serum antibodies; structure similar to IgG; found in blood, lymph, and on B cells; assists in the immune response on B cells, but function is not well-defined.

• IgE: Monomer; 0.002% of serum antibodies; found on mast cells, basophils, and in blood; causes release of histamines when bound to antigen; involved in allergic reactions and lysis of parasitic worms.

Table: Summary of Immunoglobulin Classes

Types of Adaptive Immunity

Classification of Adaptive Immunity

• Naturally acquired active immunity: Results from infection; the body produces its own antibodies and memory cells.

• Naturally acquired passive immunity: Antibodies are transferred from mother to child transplacentally or via colostrum (breast milk).

• Artificially acquired active immunity: Results from injection of vaccination (immunization); stimulates the body to produce antibodies and memory cells.

• Artificially acquired passive immunity: Results from injection of antibodies; provides immediate but short-term protection.

Example: Vaccination

Vaccination is an example of artificially acquired active immunity, where exposure to a harmless form of a pathogen stimulates the immune system to develop memory cells and antibodies, providing long-term protection against future infections.

Additional info: The notes are based on standard microbiology textbook content for Chapter 17: Adaptive Immunity, including the structure and function of immunoglobulins, cytokines, and the types of adaptive immunity.