Comprehensive Biological Concepts: Immunity, Behavior, & Reproduction

Posted on Nov 7, 2025 in Biology

The Biological Basis of Immunity

Innate Immunity

  • Invertebrates: Exoskeleton, lysozyme, hemocytes, recognition proteins.
  • Vertebrates: Skin and mucous membranes, phagocytes (neutrophils, macrophages), Toll-like receptors (TLRs), antimicrobial peptides, complement system, natural killer (NK) cells, interferons.

Inflammation and the Lymphatic System

  • Triggered by injury or infection; involves mast cells releasing histamine and cytokines.
  • Lymph nodes contain macrophages and dendritic cells for pathogen clearance and activating adaptive immunity.

Adaptive Immunity

  • B Cells: Produce antibodies (immunoglobulins) that neutralize pathogens.
  • T Cells: Recognize antigen fragments on MHC molecules; include helper T cells (activate B and T cells) and cytotoxic T cells (kill infected cells).

Lymphocyte Development and Memory

  • Diversity arises from gene segments (V, J, C).
  • Self-tolerance involves eliminating self-reactive cells.
  • Clonal selection produces effector and memory cells.

Immunological Memory

  • Primary Response: Slower and weaker.
  • Secondary Response: Faster, stronger, and provides long-term immunity.

Immune Responses and Disruptions

  • Antibody Functions: Neutralization, opsonization (marking for phagocytosis), and complement activation.
  • Cell-Mediated Response: Cytotoxic T cells kill infected cells using toxic proteins and apoptosis.
  • Immunization: Autoimmune diseases, allergies, and immunodeficiency disrupt normal immunity.
  • Misinformation about vaccines spreads false safety information.

Principles of Animal Behavior (Ethology)

Key Concepts in Behavior

  • Sensory inputs trigger behaviors.
  • Learning links experience and actions.
  • Behaviors evolve through natural selection.
  • Genetics and inclusive fitness explain behavior evolution.

Proximate vs. Ultimate Causes

  • Proximate: Explains how a behavior occurs or is modified (mechanism).
  • Ultimate: Explains why a behavior evolved for survival or reproduction (evolutionary significance).

Fixed Action Patterns and Rhythms

  • Fixed Action Patterns (FAPs): Unlearned, stereotyped sequences triggered by external cues (e.g., stickleback aggression to the color red).
  • Examples: Migration (guided by environmental cues), brood parasitism.
  • Behavioral Rhythms: Governed by circadian (daily), circannual (seasonal), and lunar cycles.

Communication

  • Signals (visual, chemical, tactile, auditory) transmit information.
  • Examples: Honeybee waggle dance, pheromones.

Learning Types

  • Imprinting: Permanent bonding during a sensitive period.
  • Associative Learning: Linking stimuli (classical and operant conditioning).
  • Cognition: Reasoning, problem solving, and social learning.

Behavior and Reproductive Success

  • Foraging involves trade-offs; genes like for influence behavior.
  • Mating Systems: Monogamy, polygyny, and polyandry.
  • Parental care is often influenced by paternity certainty.

Evolution of Altruism

  • Inclusive Fitness: Helping relatives spread shared genes.
  • Hamilton’s Rule: rB > C (relatedness × benefit > cost).
  • Kin selection favors altruism among relatives.
  • Reciprocal Altruism: Cooperation based on the expectation of future favors being returned.

Animal Reproduction and Hormonal Control

Types of Reproduction

Asexual Reproduction

  • Methods include budding, fission, fragmentation, and parthenogenesis.
  • Common in invertebrates; allows reproduction without mates.

Sexual Reproduction

  • Involves the fertilization of sperm and egg.
  • Produces genetically diverse offspring.
  • The “enigma of sex” suggests advantages like genetic variation and adaptive response to environmental changes.

The Reproductive Cycle

  • Most Animals: Seasonal cues influence reproduction.
  • Ovulation: Release of mature eggs, typically at the cycle’s midpoint.
  • Environmental Impacts: Climate change affects reproductive timing and success.

Reproductive Strategies

  • Hermaphroditism: Individuals possess both male and female organs; they can self-fertilize or mate with others.
  • Sex Change: Some species can change sex based on environmental or social cues.
  • Mating Systems: Monogamy, polygyny, and promiscuity; each involves specific evolutionary strategies (e.g., lions’ territory defense, penguins’ co-parenting).

Reproductive Organs and Structures

Male Structures

  • External: Scrotum and penis.
  • Internal: Gonads (testes), accessory glands (secrete sperm nutrients), and ducts.

Female Structures

  • External: Clitoris and labia.
  • Internal: Ovaries (produce eggs and hormones), uterine tubes, uterus, and vagina.
  • Ovaries contain follicles with oocytes.

Hormonal Regulation of Reproduction

  • Hypothalamus (GnRH): Triggers the anterior pituitary gland.
  • Pituitary Hormones: FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
  • Gonadal Hormones:
    • Testosterone: Main androgen, influences male traits.
    • Estrogens & Progesterone: Regulate the female reproductive cycle and maintain pregnancy.
  • Cycle Regulation: Hormonal interplay controls gamete production, sexual behavior, and secondary sexual characteristics.

Menstrual vs. Estrous Cycles

  • Menstrual (humans, some primates): Involves endometrial shedding (menstruation); there is no limited period of sexual receptivity.
  • Estrous: Reproductive readiness occurs during “heat”; the endometrium is reabsorbed, and sexual receptivity is limited to this period.

Human Sexual Response and Menopause

  • The sexual response involves four phases: excitement, plateau, orgasm, and resolution.
  • Both sexes experience vasocongestion and myotonia.
  • Menopause: Occurs after approximately 500 cycles (around 50 years of age).
  • It is the cessation of ovulation and menstruation.
  • Menopause is rare in animals; it possibly evolved to enhance kin care.

Common Myths vs. Biological Reality

  • Females are not always defined as “the one with female genitalia” or “the one who gives birth.”
  • Males are not necessarily “bigger,” “more aggressive,” or “XY.”
  • Sex roles and behaviors are diverse across species; there is no strict link between sex chromosomes and behavior or roles.