The Immune System and Mechanisms of Defense

The Immune System and Mechanisms of Defense

What Threats Must Our Bodies Defend Against?

The human body is constantly exposed to a variety of microscopic threats, including bacteria, viruses, and cancer cells. Some of these microorganisms are harmless or even beneficial, but pathogens are those that cause disease. Defense against these threats is essential for survival.

• Bacteria: Ubiquitous, single-celled prokaryotes; some are pathogenic.

• Viruses: Nonliving infectious particles requiring host cells to reproduce.

• Cancer cells: Abnormal cells that can evade normal regulatory mechanisms and proliferate uncontrollably.

Overview of the Body’s Defense Mechanisms

The body employs multiple layers of defense to protect against pathogens and abnormal cells. These mechanisms are categorized as barriers, nonspecific (innate) defenses, and specific (adaptive) defenses.

• Barriers: Physical and chemical means to prevent pathogen entry (e.g., skin, stomach acid, tears).

• Nonspecific defenses: General responses such as phagocytosis and inflammation.

• Specific defenses: Immune responses targeting specific pathogens, involving antibodies and T cells.

Pathogens and Disease

Types of Pathogens

Disease-causing agents can be living organisms or nonliving infectious particles.

• Bacteria: Unicellular prokaryotes; some cause diseases like pneumonia and tuberculosis.

• Fungi: Unicellular or multicellular eukaryotes; can cause infections.

• Parasites: Unicellular or multicellular eukaryotes; include protozoa and worms.

• Viruses: Nonliving particles with DNA or RNA, surrounded by a protein coat (capsid).

• Prions: Infectious proteins causing neurodegenerative diseases.

Bacteria: Single-Celled Living Organisms

Bacteria are prokaryotic cells lacking a nucleus and membrane-bound organelles. While most are harmless or beneficial, some are pathogenic and can be treated with antibiotics.

• Used in food production (e.g., yogurt, sauerkraut).

• Diseases: pneumonia, botulism, syphilis, etc.

• Antibiotics target bacterial features, such as cell wall synthesis.

Viruses: Tiny Infectious Agents

Viruses are much smaller than bacteria and cannot reproduce outside a host cell. They lack metabolic activity and consist of genetic material (DNA or RNA) within a protein coat.

• Diseases: AIDS, COVID-19, influenza, hepatitis, rabies, etc.

• Antivirals can control, but rarely cure, viral infections.

Transmissibility, Mode of Transmission, and Virulence

The risk posed by a pathogen depends on how easily it spreads (transmissibility), its mode of transmission (e.g., respiratory, fecal-oral), and its virulence (severity of disease caused).

• Epidemic: Large outbreak in a community or region.

• Pandemic: Epidemic that spreads globally (e.g., COVID-19).

The Lymphatic System: Structure and Function

Functions of the Lymphatic System

The lymphatic system is essential for immune defense, fluid balance, and fat absorption.

• Maintains blood volume.

• Transports fats from the digestive system.

• Filters foreign material to defend against infection.

Structure of the Lymphatic System

The lymphatic system is a network of vessels, similar to veins, that begin as blind-ended capillaries and eventually drain into the cardiovascular system via the right lymphatic duct and thoracic duct.

Lymph Nodes: Cleansing the Lymph

Lymph nodes are small, bean-shaped structures located along lymphatic vessels. They filter lymph, removing microorganisms, debris, and abnormal cells.

• Composed of connective tissue, macrophages, and lymphocytes.

• Act as filters, cleansing lymph as it passes through.

The Spleen: Cleansing the Blood

The spleen is the largest lymphatic organ, located in the upper left abdominal cavity. It has two main regions:

• Red pulp: Removes old and damaged red blood cells; temporary blood storage.

• White pulp: Contains lymphocytes that search for pathogens.

Thymus Gland

The thymus gland, located behind the sternum, is the site of T lymphocyte maturation. It secretes hormones (thymosin and thymopoietin) that regulate T cell development. The thymus is most active during childhood and atrophies with age.

Tonsils and Adenoids

Tonsils and adenoids are masses of lymphatic tissue that filter food and air entering the throat and nasal passages, respectively. They can become infected, leading to tonsillitis.

Innate (Nonspecific) and Adaptive (Specific) Defenses

Overview of Immune Defenses

The immune system consists of two main branches: innate (nonspecific) and adaptive (specific) defenses.

• Innate defenses: Surface barriers (skin, mucous membranes) and internal defenses (phagocytes, natural killer cells, inflammation, antimicrobial proteins, fever).

• Adaptive defenses: Humoral immunity (B cells) and cellular immunity (T cells).

First Line of Defense: Physical and Chemical Barriers

The first line of defense includes physical and chemical barriers that prevent pathogen entry.

• Skin, tears, saliva (contain lysozyme), ear wax, mucus, stomach acid, vaginal acidity, vomiting, urination, defecation, and resident microbiota.

Human Microbiota

The human microbiota consists of all microorganisms living in or on the body. Many are beneficial, providing competitive exclusion of pathogens, producing vitamins, and altering the environment to inhibit pathogens. Some can become opportunistic pathogens if host defenses are weakened.

Maintaining the Normal Microbiota

• Probiotics: Supplements containing beneficial microorganisms.

• Prebiotics: Substances that promote the growth of beneficial microbes.

Nonspecific Defenses: Second Line of Defense

When pathogens breach the first line of defense, nonspecific mechanisms respond. These include:

• Complement proteins

• Phagocytic cells (neutrophils, macrophages)

• Inflammation

• Natural killer cells

• Interferons

The Complement System

The complement system consists of at least 30 proteins that circulate in the blood in an inactive state. When activated, they:

• Mark pathogens for destruction

• Enhance inflammation

• Directly lyse invading microorganisms

Phagocytes

Phagocytes are white blood cells that engulf and digest foreign cells. Neutrophils are the first responders, while macrophages serve as scavengers and present antigens to lymphocytes.

Inflammation

The inflammatory response is triggered by tissue injury and is characterized by redness, warmth, swelling, and pain. It disposes of debris and pathogens and initiates tissue repair.

Natural Killer Cells

Natural killer cells are lymphocytes that target virus-infected and tumor cells by creating holes in their membranes and releasing destructive enzymes.

Interferons

Interferons are proteins secreted by virus-infected cells that signal neighboring cells to produce antiviral proteins, thereby inhibiting viral replication.

Fever

Fever is a systemic response to infection. Pyrogens released by macrophages reset the body's thermostat, making the environment less hospitable to pathogens and enhancing immune responses. High fever can be dangerous.

Specific Defense Mechanisms: The Third Line of Defense

The Immune Response

The immune response is characterized by specificity, memory, diversity, and the ability to distinguish self from nonself. It targets specific antigens, which are usually proteins or polysaccharides on the surface of pathogens.

• MHC proteins: Self-antigens that enable recognition of self versus nonself.

Lymphocytes: B Cells and T Cells

Lymphocytes are central to specific defenses. They originate from stem cells in the bone marrow.

• B lymphocytes (B cells): Responsible for antibody-mediated (humoral) immunity.

• T lymphocytes (T cells): Responsible for cell-mediated immunity.

B Cells: Antibody-Mediated Immunity

B cells are activated when they recognize their specific antigen, often with the help of helper T cells. They differentiate into memory cells and plasma cells, the latter of which secrete antibodies.

• Antibodies bind to antigens, neutralize pathogens, promote phagocytosis, and activate complement.

The Five Classes of Antibodies

• IgG: Most prevalent; crosses placenta.

• IgM: First produced in response.

• IgA: Found in secretions (e.g., breast milk).

• IgD: Function unclear; may serve as B cell receptor.

• IgE: Key in allergic responses.

T Cells: Cell-Mediated Immunity

T cells mature in the thymus and are classified by surface markers (CD4 or CD8). They require antigen presentation by antigen-presenting cells (APCs) such as macrophages or B cells, which display antigen with MHC molecules.

• Helper T cells (CD4): Secrete cytokines to coordinate immune response; targets of HIV.

• Cytotoxic T cells (CD8): Destroy infected or abnormal cells.

• Memory T cells: Provide long-term immunity.

Immune Memory and Immunity

Immune memory allows for a faster and stronger response upon subsequent exposures to the same antigen.

• Primary response: Slower, occurs on first exposure.

• Secondary response: Rapid and robust due to memory cells.

Active and Passive Immunity

• Active immunity: Results from exposure to antigen (infection or vaccination); produces memory cells.

• Passive immunity: Results from transfer of antibodies (e.g., maternal antibodies, antibody injections); provides immediate, short-term protection without memory.

Antibiotics and Antivirals

Antibiotics are effective against bacteria by targeting unique bacterial features. They can be narrow or broad spectrum but do not affect viruses. Antivirals are less common and typically control rather than cure viral infections.