Genetics, Immunity & Infectious Disease – Exam Notes

Genetic Control of Cell Function

DNA is the stable repository of cellular instructions; every cell carries the full genome but expresses only the genes required for its role. RNA transcribes this information, is processed in the nucleus, and transports it to the cytoplasm for protein synthesis.

Basic Genetic Terms

A recessive trait is expressed only when both alleles match, whereas a dominant trait appears with one or two dominant alleles. A carrier possesses one recessive allele without showing the trait.

Genomic Mapping

The Human Genome Project showed humans share about 99.9\% of DNA. Genetic maps estimate gene positions by inherited linkage, while physical maps measure actual base-pair distances.

Single-Gene Disorders

A single defective gene can create abnormal proteins, reduce enzyme activity, disrupt receptors, or cause unusual drug reactions. Inheritance patterns include autosomal dominant, autosomal recessive, and X-linked recessive.

X-Linked Example

Fragile X syndrome involves failure of chromatin condensation at a fragile site on the X chromosome, affects people assigned male at birth more often, and is the second leading genetic cause of intellectual disability.

Chromosomal Disorders

Numerical or structural chromosome changes lead to early pregnancy loss, malformations, and intellectual disability. Classic examples are trisomy 21 (Down), monosomy X (Turner), and polysomy X (Klinefelter).

Stress and Adaptation

Selye’s general adaptation syndrome progresses through alarm (catecholamines & cortisol release), resistance (efficient defense), and exhaustion (resource depletion and wear). Stress responses vary by person and time; chronic stress disrupts cardiovascular, gastrointestinal, immune, and neurologic functions and contributes to psychological disorders.

Measuring Stress

Assessment is indirect: blood pressure, plasma or urinary catecholamines, cortisol, and multiple pituitary or adrenal hormones.

Infectious Disease Basics

A host supports another organism’s growth; colonization denotes microbial presence; virulence reflects disease-producing capacity. Agents include prions, viruses, bacteria, rickettsiae, chlamydiae, fungi, and parasites.

Viral and Bacterial Features

Viruses are the smallest pathogens, lack cellular structure, and replicate only inside living cells. Bacteria are classified by shape and Gram staining: Gram-positive stain purple, Gram-negative stain red.

Course of Infection

Typical stages are incubation, prodrome, acute illness, convalescence, and resolution. Clinical signs may be overt or subtle.

Determinants of Infection Site

Pathogen type, portal of entry, and host immune competence dictate where disease manifests.

Immune System Overview

Innate immunity offers immediate nonspecific defense via physical barriers, chemicals, and sentinel cells, and initiates inflammation. Adaptive immunity uses specialized B and T lymphocytes that recognize specific antigens, act more slowly, but provide memory.

Antigen Presentation

Macrophages and dendritic cells process antigens and display them to \text{CD4}^+ T helper cells, triggering adaptive responses.

Adaptive Responses

Humoral immunity (B cells) targets extracellular microbes with antibodies, while cell-mediated immunity (cytotoxic T cells) eliminates infected or abnormal cells.

Immunoglobulin Classes

IgG crosses placenta and neutralizes microbes; IgA protects mucous membranes; IgM dominates early responses; IgD aids B-cell maturation; IgE mediates allergy and parasite defense.

Lymphoid Organs

Bone marrow and thymus generate and mature immune cells; peripheral tissues such as lymph nodes trap antigens and facilitate immune interactions.

Cytokine Roles

IL-1, IL-6, and TNF drive fever and acute inflammation; IL-3 and GM-CSF promote blood-cell development.

Developmental and Aging Changes

Neonates develop IgA and IgM after birth. Aging reduces thymic size, slows T-cell responses, and lowers overall immune adaptability.

Hypersensitivity Disorders

Hypersensitivity is an excessive immune reaction harming host tissue.
Type I (IgE-mediated) causes rapid allergy and anaphylaxis and responds to epinephrine.
Type II (IgG/IgM cytotoxic) targets cell-bound antigens, producing complement-mediated lysis or dysfunction.
Type III forms circulating immune complexes that deposit in vessels and tissues, provoking vasculitis (e.g., lupus, serum sickness).
Type IV is delayed T-cell–mediated injury (contact dermatitis, pneumonitis). Latex allergy may involve type I or type IV mechanisms.