Immunology – Antibodies, Complement, and Antigen Presentation

Vocabulary flashcards covering major terms from antibody structure/function, complement pathways, leukocyte trafficking, immune cell lineages, and MHC-mediated antigen processing.

Immunogenicity

The ability of a substance (antigen) to elicit an immune response.

Epitope

Specific portion of an antigen recognized and bound by an antibody or T-cell receptor.

Size (immunogenic factor)

Larger molecules are generally more immunogenic because they possess more potential epitopes.

Chemical Composition (immunogenic factor)

Copolymeric and structurally complex molecules are more immunogenic than simple homopolymers.

Foreignness

Degree to which an antigen differs from host molecules; greater genetic distance enhances immunogenicity.

Susceptibility to Processing

Antigens must be degradable and presentable to be immunogenic; non-degradable molecules make poor immunogens.

Antibody

Y-shaped glycoprotein composed of two heavy and two light chains that binds antigen.

Fab Region

Fragment antigen binding portion of antibody responsible for recognizing and binding epitopes.

Fc Region

Constant region of antibody heavy chains that mediates effector functions such as complement activation and opsonization.

Hinge Region

Flexible segment in IgG, IgA, and IgD that allows Fab arms to swivel and accommodate antigen binding.

Isotype

Antibody class determined by heavy-chain constant region (IgG, IgM, IgA, IgE, IgD).

Allotype

Allelic variant of an antibody constant region that differs between individuals of the same species.

Idiotype

Unique amino-acid sequence in the variable region of an antibody that defines antigen specificity.

Complementarity-Determining Region (CDR)

Hypervariable segments within antibody variable domains that directly contact the epitope.

Affinity

Strength of a single binding interaction between an antibody and its epitope.

Avidity

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

Opsonin

Molecule (e.g., antibody or C3b) that coats a pathogen to enhance phagocytosis.

Opsonization

Process in which opsonins mark pathogens for uptake by phagocytes.

IgM

Pentameric antibody of primary responses; highest avidity; efficient at complement fixation.

IgG

Most abundant serum antibody; high affinity; crosses placenta; fixes complement.

IgA

Dimeric antibody in mucosal secretions; important for gastrointestinal and respiratory defense.

IgE

Monomeric antibody bound to mast cells and basophils; mediates allergic reactions and parasite defense.

IgD

Monomeric antibody primarily on naïve B-cell surfaces; exact function not well understood.

Complement System

Plasma protein cascade that tags microbes for destruction and promotes inflammation.

Classical Pathway

Complement activation route triggered by antibodies bound to antigen (C1qrs complex).

Alternative Pathway

Complement activation via spontaneous C3 hydrolysis and pathogen surface factors (C3bBb convertase).

Lectin Pathway

Complement activation initiated by mannose-binding lectin–MASP complex binding microbial carbohydrates.

C3 Convertase (Classical/Lectin)

Enzyme complex C4b2a that cleaves C3 into C3a and C3b.

C5 Convertase (Classical/Lectin)

Complex C4b2a3b that cleaves C5 into C5a and C5b.

C3a

Anaphylatoxin that recruits and activates phagocytes; promotes vasodilation and vascular permeability.

C5a

Potent anaphylatoxin and chemoattractant for neutrophils and other leukocytes.

C3b

Complement fragment that acts as an opsonin and part of C5 convertase; initiates MAC formation.

C5b

Complement fragment that nucleates membrane attack complex assembly leading to cell lysis.

Membrane Attack Complex (MAC)

C5b-C9 complex that forms pores in target membranes, causing lysis.

Leukocyte Extravasation

Process by which leukocytes exit bloodstream to enter inflamed tissue.

Selectins

Endothelial adhesion molecules that mediate initial rolling of leukocytes along vessel walls.

Integrins

Leukocyte adhesion receptors that bind ICAMs to enable firm arrest during extravasation.

Diapedesis

Migration of leukocytes through endothelial cell junctions into tissue.

Lymphatic System

Network of vessels that transport lymph, antigens, and immune cells to lymph nodes.

Lymph Node

Secondary lymphoid organ where antigens encounter and activate B and T lymphocytes.

Neutrophil

Most abundant circulating granulocyte; first responder phagocyte that forms pus.

Eosinophil

Granulocyte that combats parasitic worms and contributes to allergy pathology.

Basophil

Granulocyte that releases histamine and mediates allergy and parasite defense.

Mast Cell

Tissue-resident cell related to basophils; degranulates histamine during allergic responses.

Monocyte

Circulating precursor that differentiates into macrophages or dendritic cells in tissues.

Macrophage

Phagocytic cell that ingests microbes, presents antigen, and secretes cytokines.

Dendritic Cell

Professional antigen-presenting cell that initiates adaptive immunity by activating naïve T cells.

Natural Killer (NK) Cell

Innate lymphoid cell that kills virus-infected and tumor cells lacking MHC I.

Pattern Recognition Receptor (PRR)

Innate receptor that detects conserved pathogen-associated or damage-associated molecular patterns.

PAMP (Pathogen-Associated Molecular Pattern)

Conserved microbial structure recognized by PRRs (e.g., LPS, flagellin).

DAMP (Damage-Associated Molecular Pattern)

Host molecules released from damaged cells that trigger innate responses.

Innate Immunity

Immediate, non-specific defense involving barriers, phagocytes, PRRs, and complement.

Adaptive Immunity

Antigen-specific, memory-forming defense mediated by B and T lymphocytes.

MHC Class I

Molecules on all nucleated cells presenting endogenous peptides to CD8+ cytotoxic T cells.

MHC Class II

Molecules on professional APCs presenting exogenous peptides to CD4+ helper T cells.

Endogenous Antigen Processing

Cytosolic proteins degraded by proteasome, transported by TAP into ER, and loaded onto MHC I.

Exogenous Antigen Processing

Extracellular proteins endocytosed, degraded in acidified endosomes, and loaded onto MHC II.

TAP (Transporter Associated with Antigen Processing)

ER membrane protein that translocates proteasome-derived peptides into the ER for MHC I loading.

Immunoproteasome

Modified proteasome in APCs that generates peptides optimal for MHC I binding.

ERAP (Endoplasmic Reticulum Aminopeptidase)

ER enzyme that trims peptides to optimal length for MHC I presentation.

Invariant Chain

MHC II chaperone blocking peptide groove in ER to prevent premature binding.

CLIP

Fragment of invariant chain that remains in MHC II groove until replaced by antigen peptide.

HLA Genes

Highly polymorphic human MHC gene cluster inherited en bloc as haplotypes.

Cross-Presentation

Route where dendritic cells present exogenous antigens via MHC I to activate CD8+ T cells.