The Immune System: Innate and Adaptive Defenses, Disorders, and Vaccination

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Introduction to Immunity and Disease

Viruses and Cancer

Some viruses, such as the human papillomavirus (HPV), are known to cause cancers, including cervical and anal cancers. Vaccines against HPV are available, but the duration of their effectiveness is still under investigation.

A person receiving a vaccine injection

Overview of the Immune System

Big Ideas in Immunity

Innate Immunity: The first line of defense, present in all animals, acting immediately and non-specifically against pathogens.
Adaptive Immunity: A specific, acquired response found only in vertebrates, activated after exposure to particular pathogens.
Immune System Disorders: Conditions where the immune system malfunctions, leading to disease.

Innate Immunity

Innate Immunity in Animals

All animals possess innate immunity, which provides immediate, non-specific defense against pathogens (disease-causing agents). Invertebrates rely solely on innate immunity, while vertebrates have both innate and adaptive defenses.

Barriers: Skin, mucous membranes, and secretions act as physical and chemical barriers.
Cellular Defenses: Phagocytic cells (neutrophils, macrophages), natural killer cells, and defensive proteins (e.g., interferons, complement system).

Diagram of innate external and internal barriers

Phagocytosis

Phagocytes recognize pathogens by detecting molecules common to many microbes but absent from the host. They engulf and digest pathogens using lysosomes.

Phagocytosis: Pathogen engulfed by immune cell and digested in lysosome

The Inflammatory Response

Tissue damage triggers the inflammatory response, which disinfects tissues and limits infection. Key steps include:

Release of signaling molecules from mast cells and macrophages, causing capillaries to dilate.
Capillaries become leaky, allowing neutrophils to migrate to the infection site.
Neutrophils digest bacteria and cell debris, leading to tissue healing.

Step 1: Tissue injury and signaling molecule release Step 2: Capillaries widen, neutrophils migrate Step 3: Neutrophils digest bacteria, tissue heals

Adaptive Immunity

Characteristics of Adaptive Immunity

Adaptive immunity is specific to particular pathogens and has memory, allowing for a stronger response upon re-exposure. It is mediated by lymphocytes (B cells and T cells).

Antigen: Any molecule that elicits an adaptive immune response.
Antibody: A protein produced by B cells that binds specifically to an antigen.

Comparison of innate and adaptive immune responses

The Lymphatic System

The lymphatic system is crucial for both innate and adaptive immunity. It consists of lymphatic vessels, lymph nodes, and lymph (fluid). Lymph organs are packed with white blood cells that fight infections.

Main functions: Return tissue fluid to the circulatory system and fight infection.

Diagram of the lymphatic system and lymph node structure

Lymphocytes: B Cells and T Cells

Lymphocytes originate from stem cells in the bone marrow. B cells mature in the bone marrow, while T cells mature in the thymus. They provide a dual defense:

Humoral immune response (B cells): Defense against pathogens in body fluids by producing antibodies.
Cell-mediated immune response (T cells): Defense against infected cells.

Development of B and T lymphocytes from stem cells

Antigen Recognition

B cells recognize antigens directly, while T cells require antigens to be presented by other cells. The specific part of an antigen recognized is called the epitope, and the region on the antibody or receptor that binds the epitope is the antigen-binding site.

Antibodies binding to specific epitopes on an antigen

Clonal Selection

When an antigen enters the body, only lymphocytes with receptors specific to that antigen are activated. These cells divide to form clones of effector cells (which fight the infection) and memory cells (which provide long-term immunity).

Clonal selection: formation of effector and memory cells Effector cells combat infection, memory cells provide long-term immunity

Primary and Secondary Immune Responses

The first exposure to an antigen triggers the primary immune response. Subsequent exposures activate memory cells, resulting in a faster and stronger secondary immune response.

Graph comparing primary and secondary immune responses

Herd Immunity

When a high percentage of a population is vaccinated, the spread of disease is limited, protecting even those who are unvaccinated. This is known as herd immunity.

Diagram illustrating herd immunity vs. no herd immunity

Antibody Structure and Function

Antibodies have specific antigen-binding sites that allow them to bind to antigens and mark pathogens for destruction by other immune mechanisms. They do not kill pathogens directly.

Structure of an antibody and its antigen-binding sites Mechanisms of antibody action: neutralization, agglutination, complement activation

Measuring Vaccine Effectiveness

Scientists monitor antibody levels to assess the long-term effectiveness of vaccines, such as those for HPV. Data show that antibody levels remain high for years after vaccination, but ongoing monitoring is necessary.

Graph of antibody levels after HPV vaccination

Helper T Cells

Helper T cells are activated by antigen-presenting cells and stimulate both the humoral and cell-mediated immune responses by releasing signaling molecules that activate B cells and cytotoxic T cells.

Helper T cell activation by antigen-presenting cell Helper T cell stimulates B cells and cytotoxic T cells

Cytotoxic T Cells

Cytotoxic T cells destroy infected body cells by binding to them and releasing perforin and enzymes that induce cell death, depriving pathogens of a place to multiply.

Cytotoxic T cell killing an infected cell

Immune System Disorders

HIV and AIDS

HIV infects and destroys helper T cells, leading to AIDS (acquired immunodeficiency syndrome). This impairs the immune system, making patients susceptible to infections and cancers. HIV mutates rapidly, complicating treatment.

HIV infecting a human helper T cell Global statistics on HIV infection and treatment

Self-Recognition and Autoimmunity

Lymphocytes distinguish self from nonself using major histocompatibility complex (MHC) molecules. Autoimmune disorders occur when the immune system attacks self molecules. Immunodeficiency disorders result from a lack of immune components.

Allergies

Allergies are exaggerated immune responses to harmless environmental antigens (allergens). Symptoms result from the release of histamine. Antihistamines can provide temporary relief.

Sequence of events in an allergic reaction Sensitization: Initial exposure to allergen Later exposure: Allergen triggers histamine release

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific	Specific to antigens
Response Time	Immediate	Slower (days)
Memory	None	Has memory
Main Components	Barriers, phagocytes, NK cells, complement	B cells, T cells, antibodies
Presence	All animals	Only vertebrates

Key Concepts for Review

Risks and prevention of HPV infections
Nature of innate and adaptive defenses
Steps of the inflammatory response
Structure and function of the lymphatic system
Development and function of B and T lymphocytes
Nature of antigens and antibodies
Clonal selection and immune memory
Primary vs. secondary immune responses
Herd immunity and vaccination
Immune system disorders: autoimmunity, immunodeficiency, allergies