Immunology Lecture Notes – Key Concepts (Video)

Key vocabulary terms and concise definitions drawn from the lecture notes on innate and acquired immunity, cellular players, antigen concepts, the complement system, immunoglobulins, cytokines, and immunopotentiation.

Immunology

The study of the body's defense mechanisms against foreign pathogens and aberrant self-cells, including organs, cells, and molecules of the immune system.

Immune components

Organs, cells (e.g., lymphocytes), and molecules (e.g., antibodies) that constitute the immune system.

Host-microbial relationship

Factors influencing infection, including microbial factors and host factors that determine disease outcome.

Specificity

Each pathogen causes a distinct disease, reflecting a unique host-pathogen interaction.

Koch’s postulates

Five criteria to establish causation: (1) pathogen in all cases, (2) isolated in pure culture, (3) causes disease in susceptible hosts, (4) re-isolatable from infected hosts, (5) detectable specific antibodies in host serum.

Exceptions to Koch’s postulates

Examples where postulates don’t hold: Neisseria gonorrhoeae (no good animal model) and Mycobacterium leprae (unculturable in vitro).

Infectivity

Pathogen’s ability to invade, colonize, and multiply within a host.

Reservoirs

Sources from which a pathogen maintains itself: endogenous (within the host) or exogenous (external) reservoirs.

Endogenous reservoir

Commensals or other organisms from the host that become pathogenic.

Exogenous reservoir

Pathogens sourced outside the host, including humans, animals, environment, or vectors.

Carrier

A person who carries pathogenic organisms and transmits them without showing symptoms.

Carrier types

Healthy (asymptomatic), incubatory (before symptoms), and convalescent (recovery) carriers.

Transmission routes

Ways pathogens spread: Airborne, Oral, Vector-borne, Contact (direct/indirect), and Parenteral.

Airborne transmission

Spread via droplets or aerosols that are inhaled.

Oral transmission

Ingestion of contaminated food or water.

Vector-borne transmission

Transmission by vectors such as insects (e.g., malaria via mosquitoes).

Contact transmission

Direct contact or contact with contaminated surfaces (fomites).

Parenteral transmission

Pathogens transmitted by wounds or injections bypassing the gut.

Invasiveness

Pathogen’s capacity to penetrate host tissues.

Invasive mechanisms

Strategies used by pathogens: enzymes (hyaluronidase, collagenase) and anti-phagocytic structures (capsules, M-proteins), plus intracellular survival (e.g., M. tuberculosis).

Pathogenicity

Ability of a microorganism to cause disease.

Virulence

Degree of pathogenicity or how severe the disease is.

Infective dose

Amount of pathogen required to establish infection.

MLD (Minimal Lethal Dose)

Dose that kills 100% of hosts in a given test.

LD50

Dose that kills 50% of hosts in a given test.

Exotoxin

Toxin secreted by Gram-positive bacteria that harms the host.

Endotoxin

Toxic component of Gram-negative bacterial cell walls (e.g., LPS).

Innate immunity

Non-specific natural defenses present at birth that act quickly.

Acquired (adaptive) immunity

Specific immune response developed after exposure to antigens, with memory.

Passive acquired immunity

Immunity transferred from another source (natural: transplacental, lactation; artificial: antitoxins, gamma globulins).

Active acquired immunity

Immunity generated by an individual’s own immune response after exposure to an antigen (natural infection or artificial vaccination).

T-independent antigens

Antigens (often polysaccharides) that stimulate B cells without T cell help; often multivalent.

T-dependent antigens

Antigens (usually proteins) requiring T cell help to activate B cells.

Hapten

Incomplete antigen that is antigenic but not immunogenic unless attached to a larger carrier protein.

Immunogenic vs antigenic

Immunogenic substances elicit an immune response; antigens are recognized by the immune system but may not be immunogenic on their own.

Autoantigen

An antigen derived from the host’s own tissues.

Isoantigen

Antigen present in genetically identical individuals (e.g., identical twins).

Alloantigen

Antigen present in genetically different members of the same species.

Xenoantigen

Antigen from a different species.

Epitope

The part of an antigen that is recognized by an antibody (antibody-binding site).

Valency

Number of epitopes available on an antigen to which antibodies can bind.

Affinity

Strength of the attraction between a single antigen epitope and an antibody combining site.

Avidity

Overall strength of multiple binding interactions between a multivalent antigen and antibody.

Immunogenicity

Ability of a substance to provoke an immune response.

Factors influencing immunogenicity: Foreignness

How different the substance is from the host, increasing immune recognition.

Factors influencing immunogenicity: Molecular size

Larger molecules are generally more immunogenic.

Factors influencing immunogenicity: Chemical complexity

More complex molecules (e.g., proteins) tend to be more immunogenic.

Factors influencing immunogenicity: Physical form

Particulate antigens are usually more immunogenic than soluble ones.

Factors influencing immunogenicity: Degradability

Easily degradable antigens are typically more immunogenic.

Central lymphoid organs

Organs where immune cells develop: bone marrow and thymus.

Peripheral lymphoid organs

Organs where immune responses occur: lymph nodes, spleen, MALT.

Spleen

Filters blood, removes aged red cells, conducts phagocytosis and immunoglobulin synthesis.

Mucosa-associated lymphoid tissue (MALT)

Lymphoid tissue in mucosal sites contributing to mucosal immunity.

Antigen

A substance capable of stimulating an immune response; forms include complete immunogens and haptens.

Complete antigen (immunogen)

Antigen that induces an immune response and is reactive with specific antibodies or lymphocytes.

Incomplete antigen (hapten)

Small molecule that is antigenic but immunogenic only when coupled to a larger carrier.

Autoantigen/ Isoantigen/ Alloantigen/ Xenoantigen

Different classes of antigens based on origin: self, genetically identical, genetically different within species, or from a different species.

Determinant groups of antigen (epitopes)

Sites on antigens that antibodies recognize; determine antigen specificity and may be multivalent.

Affinity constant (paratope-antigen binding)

Measure of the strength of a single antigen-antibody interaction.

Immunogenicity determinants

Factors like foreignness, size, complexity, cross-reactivity, and degradability influence immunogenicity.

T-independent antigens vs T-dependent antigens

T-independent: activate B cells without T cells (often polysaccharides); T-dependent: require T cell help (proteins).

B cells vs T cells

B cells produce antibodies; T cells mediate cellular immunity and help B cells; both have distinct receptors and functions.

MHC/HLA

Major histocompatibility complex; gene complexes that present processed antigens to T cells (Class I to CD8, Class II to CD4; Class III includes complement proteins).

Class I MHC

MHC molecules expressed on all nucleated cells; present antigens to CD8+ T cells.

Class II MHC

MHC molecules on antigen-presenting cells (B cells, dendritic cells, Th); present to CD4+ T cells.

Class III MHC

Genetic region encoding several immune-related proteins, including some complement components.

Immunoglobulin (Ig)

Antibodies; glycoproteins produced by plasma cells derived from B cells; recognize specific antigens.

Light chain (κ and λ)

Two types of light chains in Ig molecules; paired with heavy chains to form the antigen-binding site.

Heavy chain types (IgG, IgA, IgM, IgD, IgE)

Different constant regions defining antibody classes with distinct functions and distributions.

Fab and Fc fragments

Fab contains the antigen-binding region; Fc mediates effector functions (e.g., complement, Fc receptors).

Hinge region

Flexible region in Ig molecules allowing binding to antigens with multiple epitopes.

IgG

Major serum Ig (75%); crosses placenta; fixes complement (except IgG4); predominant in secondary responses; opsonizes.

IgA

Present in serum and secretions; secretory IgA protects mucosal surfaces; forms secretory piece.

IgM

First antibody produced in primary response; pentameric; strong complement activator.

IgD

Low-abundance Ig; functions as B cell receptor on naive B cells.

IgE

Reagin antibody; mediates immediate hypersensitivity; binds to mast cells and basophils.

Opsonization

Coating of a pathogen with antibodies/complement to enhance phagocytosis.

Complement system

Group of heat-labile proteins that assist immunity; activation leads to lysis, chemotaxis, opsonization, and inflammation.

Classical pathway (complement)

Activation via antibodies (IgG/IgM) binding to antigen, leading to sequential C1–C9 activation.

Alternative pathway (complement)

Activation triggered directly by microbial surfaces, independent of antibodies.

Membrane attack complex (MAC)

Final assembly (C5b–C9) that forms pores in target cell membranes.

MHC/HLA roles

Antigen presentation, graft rejection/compatibility, disease associations, and diagnostic use.

Immunopotentiation

Enhancement of immune responses, often using adjuvants or immune stimulants.

Adjuvants

Substances that boost immune responses to an antigen (e.g., Freund’s adjuvant, aluminum hydroxide, BCG).

Freund’s adjuvant (incomplete vs complete)

Incomplete adjuvant: oil–water mix; complete adjuvant adds bacterial components to enhance response.

Transfer factor

A preparation claimed to transfer immune knowledge between cells to boost response.

Interferons (IFN)

Cytokines that inhibit viral replication and modulate immune responses (Type I: IFN-α/β; Type II: IFN-γ).

Interleukins (IL-1, IL-2, IL-4, IL-5, IL-6)

Cytokines with specific roles in fever, T and B cell activation, and acute-phase responses.

TNF-α/TNF-β

Cytokines with roles in inflammation, cytotoxicity, and tumor cell killing.

GM-CSF

Granulocyte–macrophage colony-stimulating factor; promotes growth of white blood cells.

NK cells (Natural Killer)

Lymphocytes that kill virus-infected or tumor cells without antibody help; part of innate immunity.

ADCC (antibody-dependent cellular cytotoxicity)

Killing of antibody-coated target cells by effector cells (e.g., NK cells) via Fc receptors.

Cytokines in T cell help

IL-2, IL-4, IL-5 mediate T cell proliferation and B cell help; Th cells orchestrate responses.

Immunosuppression

Deliberate reduction of immune activity to prevent rejection, treat autoimmune disease, or control inflammation.

Immunosuppressive methods

Physical removal of immune organs, radiation, or drugs that inhibit T/B cells (e.g., anti-thymocyte serum, methotrexate, cyclosporine, steroids).

Monoclonal antibodies

Antibodies derived from a single B cell clone with identical specificity; used in research and therapy.