Adaptive Immunity: Specific Defenses of the Host

Adaptive Immunity: Specific Defenses of the Host

Dual Nature of the Adaptive Immune System

The adaptive immune system is characterized by its ability to specifically recognize and respond to foreign substances, known as antigens. It consists of two main branches: humoral immunity and cellular immunity, each with distinct mechanisms and roles in host defense.

• Humoral Immunity: Mediated by B lymphocytes (B cells), which produce antibodies that target extracellular pathogens and toxins.

• Cellular Immunity: Mediated by T lymphocytes (T cells), which recognize antigenic peptides presented by phagocytic cells and are effective against intracellular pathogens such as viruses and certain bacteria.

B cells are created and mature in the red bone marrow, while T cells mature in the thymus. Both cell types migrate to lymphoid tissues, where they participate in immune responses.

Comparison of Humoral and Cellular Immunity

Humoral immunity combats threats outside cells, such as extracellular bacteria and toxins, while cellular immunity targets antigens that have already entered cells, including viruses and intracellular bacteria.

• Humoral Immunity: Effective against pathogens in body fluids.

• Cellular Immunity: Effective against pathogens within host cells.

Cytokines: Chemical Messengers of Immune Cells

Cytokines are protein messengers produced by immune cells in response to stimuli. They regulate the intensity and duration of immune responses by acting on various target cells.

• Interleukins (ILs): Facilitate communication between leukocytes.

• Chemokines: Induce migration (chemotaxis) of leukocytes.

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

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

• Hematopoietic cytokines: Control development of red and white blood cells.

Overproduction of cytokines can result in a cytokine storm, which is harmful to the host.

Key Cytokines and Their Roles

Cytokine	Effect	Target Cell(s)	Source
IL-4	Proliferate TH2 cells; class switching to IgE	Naïve TH; B cells	TH2
IL-12	Stimulates growth/function of TH1 cells; stimulates IFN-γ and TNF-α	Naïve TH; NK cells	Dendritic cells, macrophages, neutrophils
IL-17	Inflammation	Neutrophils	TH17
IL-22	Stimulates epithelial cells to make antimicrobial proteins	Epithelial cells	TH17
IFN-γ	Promote phagocytosis; activate macrophages and humoral response	CTL, macrophages	TH1
Chemokines	Chemotaxis	Neutrophils	Varies; macrophages, epithelial cells
TNF-α	Inflammation	Tumor cells	Macrophages, TH, NK cells
GM-CSF	Increase macrophages and granulocytes	Myeloid stem cells	Macrophages, T cells, NK cells

Antigens and Epitopes

Antigens are substances that provoke an immune response, typically by stimulating the production of antibodies. They are usually components of invading microbes, such as capsules, cell walls, flagella, fimbriae, toxins, viral capsids, and spikes. Nonmicrobial antigens include egg white, pollen, and cell surface molecules.

• Epitopes (Antigenic Determinants): Specific regions on an antigen that antibodies recognize and bind to.

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

Humoral Immunity: Antibodies (Immunoglobulins)

Antibodies are compact, soluble proteins called immunoglobulins (Ig) that specifically recognize and bind to antigens, targeting them for destruction. Each antibody has a valence, which is the number of antigen-binding sites.

• Structure: Four protein chains form a Y shape—two identical light chains and two identical heavy chains joined by disulfide links.

• Variable (V) regions: Located at the ends of the arms; bind epitopes.

• Constant (Fc) region: The stem, identical for a particular Ig class.

• Classes: IgG, IgM, IgA, IgD, IgE.

Immunoglobulin Classes

There are five main classes of immunoglobulins, each with distinct structures and functions:

Class	Structure	% Total Serum Antibody	Location	Known Functions
IgG	Monomer	80%	Blood, lymph, intestine	Enhances phagocytosis, neutralizes toxins/viruses, protects fetus
IgM	Pentamer (with J chain)	6%	Blood, lymph, B cell surface	First response, agglutination, complement activation
IgA	Dimer (with secretory component)	13%	Secretions (tears, saliva, mucous, milk)	Local protection in mucous membranes
IgD	Monomer	0.02%	B cell surface, blood, lymph	Initiates immune response on B cells
IgE	Monomer	0.002%	Bound to mast cells, basophils, blood	Allergic reactions, lysis of parasitic worms

Types of Adaptive Immunity

Adaptive immunity can be acquired naturally or artificially, and can be active or passive:

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

• Naturally acquired passive immunity: Antibodies are transferred from mother to fetus via placenta or to infant via breast milk.

• Artificially acquired active immunity: Results from vaccination; the body produces antibodies in response to introduced antigens.

• Artificially acquired passive immunity: Injection of preformed antibodies.

Summary Table: Types of Adaptive Immunity

Type	How Acquired	Key Features
Naturally Acquired Active	Infection	Body produces antibodies and lymphocytes
Naturally Acquired Passive	Placenta, breast milk	Antibodies transferred from mother
Artificially Acquired Active	Vaccination	Body produces antibodies in response to vaccine
Artificially Acquired Passive	Injection	Preformed antibodies introduced

Additional info:

Adaptive immunity is highly specific and involves memory, allowing for a more rapid and effective response upon subsequent exposure to the same antigen. The dual nature of the system ensures comprehensive protection against a wide variety of pathogens.