Chapter 7: Immunology — Innate Defense, Adaptive Immunity, and Vaccines

The immune system is the body’s defense network — a sophisticated interplay of physical barriers, circulating cells, and molecular signals that distinguishes self from non-self and eliminates pathogens without destroying host tissue.

Innate vs Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Speed	Immediate (minutes–hours)	Slow (days–weeks)
Specificity	Non-specific (PAMPs/pattern recognition)	Highly specific (antigen-specific receptors)
Memory	None	Yes — basis of vaccines
Key cells	Neutrophils, macrophages, NK cells, dendritic cells	T lymphocytes, B lymphocytes
Soluble factors	Complement, interferons, cytokines	Antibodies (immunoglobulins)

Pattern Recognition Receptors (PRRs): innate immune cells recognize conserved microbial patterns called PAMPs (pathogen-associated molecular patterns) via toll-like receptors (TLRs), triggering inflammation.

Adaptive Immunity: T Cells and B Cells

T lymphocytes (mature in thymus):

• CD4⁺ helper T cells (Th): orchestrate immune responses; activate B cells and cytotoxic T cells; release cytokines (IL-2, IL-4, IFN-γ). HIV targets CD4⁺ cells.
• CD8⁺ cytotoxic T cells (CTL): kill virus-infected cells and tumor cells via perforin/granzyme
• Regulatory T cells (Treg): suppress immune responses; prevent autoimmunity

B lymphocytes (mature in bone marrow):

• Upon activation, differentiate into plasma cells (antibody factories) and memory B cells
• Require T cell help (via CD40L–CD40 interaction) for class switching

Antibody Structure

Antibodies (immunoglobulins, Ig) are Y-shaped glycoproteins with:

• Variable regions: antigen-binding sites (Fab region)
• Constant region: determines antibody class and effector function (Fc region)

Isotype	Primary Role
IgG	Most abundant; crosses placenta; secondary response
IgM	First antibody produced (primary response); pentamer
IgA	Mucosal immunity (gut, respiratory tract, breast milk)
IgE	Allergic responses; defense against parasites
IgD	B cell receptor; role in B cell activation

MHC (Major Histocompatibility Complex)

• MHC class I (on all nucleated cells): presents endogenous antigens to CD8⁺ T cells — “self surveillance”
• MHC class II (on APCs: macrophages, dendritic cells, B cells): presents exogenous antigens to CD4⁺ T cells

Vaccines: Active and Passive Immunity

Active immunity: the immune system generates its own response (natural infection or vaccination). Long-lasting.

Passive immunity: preformed antibodies transferred to an individual (maternal IgG, IVIG, antitoxin). Immediate but temporary (half-life ~3–4 weeks).

Types of vaccines:

• Live attenuated: MMR, varicella, yellow fever — strong, durable immunity; contraindicated in immunosuppressed
• Inactivated/killed: influenza (injectable), hepatitis A, polio (IPV) — safer but require boosters
• Subunit/recombinant: hepatitis B, acellular pertussis, HPV — highly purified, fewer side effects
• mRNA vaccines: COVID-19 (Pfizer-BioNTech, Moderna) — instruct cells to produce antigen protein

Autoimmune Diseases

When self-tolerance breaks down, the immune system attacks host tissue:

Disease	Autoantibody/Mechanism	Affected Tissue
Rheumatoid arthritis	Anti-CCP, RF	Synovium → joints
SLE	Anti-dsDNA, Anti-Smith	Multi-system (kidney, skin, CNS)
Type 1 DM	Anti-GAD, Anti-IA2	Pancreatic β cells
Hashimoto’s thyroiditis	Anti-TPO, Anti-thyroglobulin	Thyroid
Myasthenia gravis	Anti-AChR	Neuromuscular junction

Key Checklist

• Distinguishes innate from adaptive immunity by speed, specificity, and key cells
• Explains MHC class I and II and their roles in antigen presentation
• Describes how vaccines produce active immunity and contrasts vaccine types