Innate and Adaptive Immunity: Mechanisms, Cells, and Vaccines

Study Guide - Smart Notes

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Innate Immunity: Nonspecific Defenses of the Host

First Line of Defense

The first line of defense consists of physical and chemical barriers that prevent the entry of pathogens into the body. These defenses are non-specific, meaning they act against a wide variety of potential invaders.

Structural Barriers: The skin (approximately 2 square meters) and mucous membranes (about 400 square meters) provide a physical barrier to pathogens.
Mechanical Defenses: The mucociliary system traps and moves particles out of the respiratory tract, while urine flow flushes microbes from the urinary tract.
Biochemical Barriers: Substances such as hydrochloric acid (HCl) in the stomach, lysozyme in saliva and tears, fatty acids on the skin, and bile in the intestine inhibit microbial growth.

Inflammation

Inflammation is a complex response to tissue injury or infection, characterized by several cardinal symptoms and physiological changes.

Causes: Triggered by tissue damage, infection, or immune reactions. Vasodilation increases blood flow to the affected area.
Symptoms: Redness, heat, swelling, pain, and sometimes loss of function.
Phagocyte Migration: White blood cells move to the site of infection through a process called diapedesis (movement through vessel walls).

Chemical Mediators of the Immune System

Chemical mediators are molecules that regulate immune responses and inflammation.

Cytokines: Proteins that modulate the function of immune cells.
Histamines: Cause vasodilation and increased vascular permeability.
Interleukin-1 (IL-1): Induces fever and activates immune cells.

Fever

Fever is a systemic response to infection, mediated by the hypothalamus in response to pyrogens such as IL-1.

Role of Hypothalamus: Regulates body temperature; IL-1 signals the hypothalamus to raise the set point, resulting in fever.
Effects: Fever can inhibit microbial growth and enhance immune responses.

Phagocytosis

Phagocytosis is the process by which certain cells engulf and destroy pathogens.

Chemotaxis and Adherence: Phagocytes are attracted to infection sites and bind to microbes.
Attachment and Engulfment: The phagocyte membrane surrounds and engulfs the microbe, forming a phagosome.
Phagosome Formation: The microbe is enclosed in a membrane-bound vesicle.
Lysosome Fusion: Lysosomes fuse with the phagosome to form a phagolysosome, where enzymes digest the microbe.

Leukocytes (White Blood Cells)

Leukocytes are the main cellular components of the immune system, classified as granulocytes or agranulocytes.

Granulocytes: Neutrophils, eosinophils, basophils (contain granules in their cytoplasm).
Agranulocytes: Monocytes (which differentiate into macrophages), dendritic cells, lymphocytes (T and B cells).

Adaptive Immunity: Specific Defenses of the Host

Antigens

Antigens are substances that can provoke an immune response. They are recognized as 'non-self' by the immune system.

Self vs. Non-self: The immune system distinguishes between the body's own molecules and foreign antigens.
Types: Proteins are generally more immunogenic than polysaccharides.
Antigen Processing Cells: Cells such as dendritic cells, macrophages, and B cells present antigens to lymphocytes.

Antibodies (Immunoglobulins, Ig)

Antibodies are globulin proteins produced by B cells that specifically bind to antigens.

Structure: Y-shaped molecules with variable regions that bind antigens.
Functions: Agglutination (clumping of antigens), opsonization (enhancing phagocytosis), neutralization (blocking pathogen activity).

B-Lymphocytes (B Cells)

B cells mature in the bone marrow and are responsible for antibody-mediated immunity.

Plasma Cells: Differentiated B cells that secrete large amounts of antibodies.
Clonal Selection: Only B cells with receptors specific to an antigen are activated and proliferate.
Memory B Cells: Long-lived cells that mediate a rapid secondary response upon re-exposure to the antigen.

Primary vs. Secondary Immune Responses

The immune system responds more rapidly and strongly upon second exposure to an antigen.

Primary Response: Initial exposure; slower, lower antibody levels.
Secondary Response: Subsequent exposures; faster, higher antibody levels due to memory cells.
Booster Shots: Vaccines given to stimulate the secondary response and increase immunity.

T-Lymphocytes (T Cells)

T cells mature in the thymus and are involved in cell-mediated immunity.

Helper T Cells (Th): Activate B cells, cytotoxic T cells, and other immune cells by releasing cytokines.
Cytotoxic T Cells (Tc): Kill infected or abnormal cells using perforins (form pores) and granzymes (induce apoptosis).

Immunization and Vaccines

Bacterial Vaccines

Vaccine	Pathogens	Type of Components
DTaP	Diphtheria, Pertussis, Tetanus	Toxoids (inactivated toxins), acellular antigens (no live bacteria)
Hib	Haemophilus influenzae type b	Conjugated polysaccharide and protein
Strep pneumoniae	Streptococcus pneumoniae	Acellular antigens conjugated with protein
Meningitis	Various bacteria	Conjugated or subunit vaccines

Viral Vaccines

Vaccine	Pathogens	Type of Components
Polio (Sabin)	Poliovirus	Live attenuated virus
Polio (Salk)	Poliovirus	Killed virus
Influenza	Influenza virus	Live attenuated or killed virus
MMR	Measles, Mumps, Rubella	Live attenuated viruses
Chickenpox	Varicella-zoster virus	Live attenuated virus
Hepatitis A, B	Hepatitis A, B viruses	Killed virus (A), antigen component (B)
HPV	Human papillomavirus	Antigen component

Toxoids

Toxoids are inactivated toxins that remain antigenic (capable of inducing an immune response) but are no longer toxic.

Types of Immunity

Immunity can be classified based on how it is acquired and whether it is active or passive.

Type	How Acquired	Duration	Example
Natural Active	Infection/disease	Long-lived	Recovery from measles
Artificial Active	Vaccination	Long-lived	MMR vaccine
Natural Passive	Maternal antibodies (placenta, breast milk)	Short-lived (1-4 months)	IgG from mother to fetus
Artificial Passive	Injection of antibodies (gamma globulin)	Short-lived	Rabies immune globulin

Additional Key Concepts

Interferons: Proteins produced by cells in response to viral infection; they inhibit viral replication in neighboring cells.
Complement System: A group of proteins that, when activated, enhance phagocytosis, lyse pathogens, and promote inflammation.
Opsonization vs. Neutralization: Opsonization coats pathogens to enhance phagocytosis; neutralization blocks pathogen activity by binding to critical sites.
Clonal Selection: The process by which an antigen selects and activates specific B or T cell clones for proliferation.

Sample Review Questions

What are the general symptoms of inflammation?
What are the general effects of the body’s chemical mediators?
What is the effect of interleukin-1 on the hypothalamus?
What are the steps in the sequence of phagocytosis?
Which leukocytes are granulocytes?
Distinguish between monocytes and macrophages.
How does interferon interfere with viral replication?
What are the characteristics of a substance that make it an antigen?
Compare the relative immunogenic strengths of proteins vs. polysaccharides.
Describe an immunoglobulin – structure and function.
What is the difference between the antigen-antibody reactions known as opsonization and neutralization?
Where do B lymphocytes and T lymphocytes mature into functioning cells in the body?
What is meant by B cell/T cell clones? What selects which clone is activated by an antigen?
What are plasma cells and memory cells?
Distinguish the differences in the roles of cytotoxic and helper T cells.
What are perforins and granzymes?
Describe the differences between the primary and secondary immune reactions. How are booster shots related to these reactions?
What pathogens are included in the DTaP vaccine? What type of components are in the vaccine?
What pathogens are included in the MMR vaccine? What type of components are in the vaccine?
What is meant by “attenuated”?
What is the difference between active and passive immunization?
What are the advantages and disadvantages of passive immunization?
What are the 4 types of immunity? Be able to recognize examples of each type.

Additional info: Where content was brief or listed, academic context and definitions were added for clarity and completeness. Tables were constructed to summarize vaccine types and immunity classifications.