Immune System and Mechanisms of Defense: Human Biology Study Notes

Immune System and Mechanisms of Defense

9.1 Pathogens Cause Disease

The human body is constantly exposed to various pathogens, which are agents that cause disease. Understanding their characteristics and how they interact with the body is essential for human biology.

• Bacteria: Single-celled organisms with a single chromosome. Some bacteria are beneficial, aiding in vitamin production and controlling harmful bacteria, while others are pathogenic, causing diseases such as pneumonia and tuberculosis. Bacterial infections are typically treated with antibiotics.

• Viruses: Very small infectious agents composed of genetic material surrounded by a protein capsid, sometimes with an envelope. Viruses lack cellular structures and cannot reproduce independently, requiring host cells for replication. Diseases caused by viruses include AIDS, hepatitis, and chickenpox. Antibiotics are ineffective against viruses.

• Prions: Misfolded proteins that can induce misfolding in normal proteins, leading to diseases such as mad cow disease. Prions are resistant to conventional sterilization methods and currently have no cure.

• Transmissibility: Refers to how easily a pathogen spreads from person to person. Virulence is the severity of the disease caused. Epidemics are localized outbreaks, while pandemics are widespread.

• Example: The Zika virus, transmitted by mosquitoes, can cause microcephaly, a condition where the brain is underdeveloped.

9.2 The Lymphatic System

The lymphatic system is a network of vessels and organs that helps maintain fluid balance, transports fats, and defends against infections.

• Lymphatic Vessels: Begin as blind-ended capillaries near cells and blood vessels. They transport lymph, a fluid containing white blood cells, proteins, fats, and bacteria.

• Lymph Nodes: Cleanse lymph by removing microorganisms and debris. Macrophages and lymphocytes are key cells involved in this process.

• Spleen: The largest lymphatic organ, responsible for cleansing blood and controlling the quality of red blood cells. Contains red pulp (macrophages) and white pulp (lymphocytes).

• Thymus Gland: Site of T cell maturation, secreting hormones thymosin and thymopoietin. Largest during childhood.

• Tonsils and Adenoids: Protect the throat and nasal passages by filtering microorganisms.

9.3 Keeping Pathogens Out

The body employs several barriers to prevent pathogen entry.

• Skin: Acts as a physical and chemical barrier, with acidic pH and antimicrobial compounds like dermicidins.

• Tears, Saliva, Earwax: Contain lysozymes that kill bacteria.

• Mucus: Traps pathogens in the digestive tract and airways; cilia sweep mucus out.

• Acids: Stomach and vaginal acids destroy many pathogens.

• Microbiota: Beneficial bacteria compete with harmful organisms, reducing infection risk. Lactobacillus is an example.

• Probiotics: Live microorganisms consumed for health benefits. Prebiotics: Dietary supplements that promote beneficial bacteria growth.

9.4 Nonspecific Defenses (Innate Defenses)

Nonspecific defenses respond to all types of pathogens and are present from birth.

• Complement System: Consists of 30 plasma proteins that form complexes to lyse bacteria and trigger inflammation.

• Phagocytes: White blood cells that engulf and digest pathogens. Types include neutrophils, monocytes/macrophages, and eosinophils.

• Inflammation: Characterized by redness, warmth, swelling, and pain. Initiated by histamines and increased blood flow.

• Natural Killer Cells: Lymphocytes that destroy tumor and virus-infected cells by releasing chemicals.

• Interferons: Proteins that stimulate healthy cells to resist viral infection.

• Fever: Macrophages release pyrogens, raising body temperature to inhibit pathogen growth.

9.5 Specific Defenses (Adaptive Immunity)

Specific defenses recognize and target particular pathogens, involving antibodies and T cells. They have memory and diversity.

• Antigens: Molecules recognized as foreign by the immune system. MHC proteins are self-markers unique to each individual.

• Lymphocytes: White blood cells including B and T cells. B cells mediate antibody immunity; T cells mediate cellular immunity.

• B Cells: Produce antibodies that neutralize pathogens. Memory B cells enable rapid response upon re-exposure.

• T Cells: Attack infected or abnormal cells. CD4 cells become helper T cells; CD8 cells become cytotoxic T cells.

• Antibody Structure: Y-shaped molecules with two heavy and two light chains, each with antigen-binding sites.

• Immunoglobulin Classes: IgG, IgM, IgA, IgD, IgE, each with specific roles in immunity.

• Helper T Cells: Coordinate immune response by secreting cytokines.

• Cytotoxic T Cells: Directly attack infected cells using perforin and granzymes.

• Memory T Cells: Enable rapid response upon subsequent exposure to the same antigen.

9.6 Immune Memory

The immune system retains memory of pathogens, allowing for faster and more effective responses upon re-exposure.

• Primary Immune Response: Initial exposure to an antigen triggers B and T cell activation, with a lag time of 3-6 days.

• Secondary Immune Response: Subsequent exposures activate memory cells, resulting in a faster and stronger response.

• Memory Cells: Long-lived cells that provide lasting immunity.

9.7 Medical Assistance

Medical interventions can enhance or support the immune system.

• Active Immunization: Vaccines stimulate the immune system to develop immunity in advance.

• Passive Immunization: Administration of antibodies for immediate, short-term protection.

• Monoclonal Antibodies: Laboratory-produced antibodies used in research, diagnostics, and therapy (e.g., cancer treatment).

• Antibiotics: Target bacterial infections by exploiting differences between bacterial and human cells.

• Tissue Rejection: Immune response against transplanted tissues; managed with immunosuppressive drugs.

Inappropriate Immune System Activity

Sometimes the immune system responds inappropriately, leading to allergies or autoimmune diseases.

• Allergies: Overreaction to harmless substances (allergens), often involving IgE antibodies and histamine release. Severe reactions can cause anaphylactic shock.

• Autoimmune Disorders: Immune system attacks the body's own cells. Examples include type 1 diabetes, lupus, and rheumatoid arthritis.

9.10 Immune Deficiency

Immune deficiency occurs when the immune system is unable to mount an effective response.

• Severe Combined Immunodeficiency Disease (SCID): Inherited condition with insufficient lymphocytes.

• AIDS: Caused by HIV, which targets helper T cells (CD4). HIV is a retrovirus that integrates its DNA into host cells, leading to progressive immune deficiency.

• Transmission: HIV spreads through blood, semen, breast milk, and vaginal secretions.

• Phases: Acute infection, chronic latency, and AIDS (marked by low helper T cell count and opportunistic infections).

• Testing: Includes antibody, antigen/antibody, and nucleic acid tests.

• Treatments: Fusion inhibitors, reverse transcriptase inhibitors, integrase inhibitors, and protease inhibitors. No complete cure, but pre-exposure (PrEP) and post-exposure (PEP) prophylaxis are available.

Summary Table: Immune System Defense Mechanisms

Defense Type	Main Components	Function
Nonspecific (Innate)	White blood cells, complement proteins, inflammation, fever	General response to all pathogens
Specific (Adaptive)	B cells, T cells, antibodies	Targeted response to specific pathogens; memory
Inappropriate Activity	IgE, cytotoxic T cells	Allergies, autoimmune diseases
Immune Deficiency	Helper T cells (CD4), HIV	Reduced immune response; AIDS