Adaptive Immunity: Structure, Function, and Types

Adaptive Immunity Part 2

Overview of Adaptive Immunity

Adaptive immunity is the third line of defense in the immune system, providing specific responses to particular pathogens. It is acquired through infection or vaccination and is characterized by its ability to "remember" previous encounters with pathogens, resulting in a more rapid and effective response upon subsequent exposures.

• Primary response: The initial immune reaction to a specific foreign substance.

• Secondary response: Enhanced and faster response upon re-exposure to the same antigen, due to immunological memory.

Dual Nature of the Adaptive Immune System

Humoral Immunity

Humoral immunity is mediated by antibodies produced by B lymphocytes (B cells). It is primarily effective against extracellular pathogens and toxins.

• B cells: Lymphocytes that originate and mature in the red bone marrow.

• Antibodies: Proteins that specifically recognize and bind to antigens.

• Target: Invaders and threats outside cells, such as extracellular bacteria, toxins, and viruses before they enter host cells.

Example: Neutralization of bacterial toxins by antibodies in the bloodstream.

Cellular Immunity (Cell-Mediated Immunity)

Cellular immunity is mediated by T lymphocytes (T cells) and is crucial for targeting intracellular pathogens.

• T cells: Lymphocytes that mature in the thymus.

• T cell receptors (TCRs): Surface molecules that recognize antigenic peptides presented by phagocytic cells.

• Cytokines: Chemical messengers secreted by T cells upon antigen recognition, including interleukins and interferons.

• Target: Antigens within infected cells, such as viruses and some intracellular bacteria (Mycobacterium leprae, Listeria monocytogenes).

Example: Destruction of virus-infected cells by cytotoxic T lymphocytes.

Cytokines: Chemical Messengers of Immune Cells

Types and Functions of Cytokines

Cytokines are signaling molecules produced by immune cells in response to stimuli. They regulate the intensity and duration of immune responses.

• Interleukins (ILs): Mediate communication between leukocytes.

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

• Interferons (IFNs): Interfere with viral replication in host cells.

• Tumor Necrosis Factor alpha (TNF-α): Involved in inflammation and autoimmune disease processes.

• Hematopoietic cytokines: Regulate the development of blood cells from stem cells.

Note: Overproduction of cytokines can result in a cytokine storm, which may cause tissue damage.

Antigens and Haptens

Definition and Properties

• Antigens: Substances (often components of microbes or foreign materials) that elicit the production of antibodies.

• Haptens: Small molecules that are not immunogenic by themselves but can provoke an immune response when attached to larger carrier molecules.

Example: Penicillin acts as a hapten and can trigger allergic reactions when bound to proteins in the body.

Humoral Immunity: Antibodies (Immunoglobulins)

Structure and Function

• Immunoglobulins (Ig): Globular proteins that function as antibodies.

• Valence: The number of antigen-binding sites on an antibody molecule.

• Bivalent antibodies: Antibodies with two antigen-binding sites (e.g., IgG).

Classes of Immunoglobulins

Class	Structure	Serum %	Main Locations	Functions
IgG	Monomer	80%	Blood, lymph, intestine	Crosses placenta, protects fetus, triggers complement, enhances phagocytosis, neutralizes toxins/viruses
IgM	Pentamer (5 monomers + J chain)	6%	Blood vessels	First antibody produced, causes clumping of cells/viruses, short-lived
IgA	Monomer (serum), Dimer (secretions)	13%	Mucous membranes, saliva, tears, breast milk	Prevents microbial attachment to mucous membranes
IgD	Monomer	0.02%	Blood, lymph, B cell surface	Assists in immune response on B cells; function not well-defined
IgE	Monomer	0.002%	Mast cells, basophils, blood	Triggers histamine release, involved in allergic reactions, lysis of parasitic worms

Additional info: The structure of immunoglobulins determines their function and distribution in the body.

Types of Adaptive Immunity

Classification Based on Acquisition

• Naturally acquired active immunity: Immunity developed after exposure to antigens through infection.

• Naturally acquired passive immunity: Transfer of antibodies from mother to child via placenta (transplacental) or breast milk (colostrum).

• Artificially acquired active immunity: Immunity induced by vaccination (injection of antigens).

• Artificially acquired passive immunity: Immunity provided by injection of preformed antibodies (e.g., antiserum).

Example: Receiving a measles vaccine provides artificially acquired active immunity; receiving antivenom after a snake bite provides artificially acquired passive immunity.