quick reference · USMLE Step 1
📖 Basic Immunology Concepts
A plain-language glossary of the core terms you'll meet across every immunology chapter — innate immunity, adaptive immunity, hypersensitivity, immunodeficiency, transplant, and diagnostics. Term on the left, simple meaning on the right.
All
Innate Immunity
Adaptive Immunity
Cells & Organs
Antibodies & Complement
Hypersensitivity & Autoimmunity
Immunodeficiency
Transplant & Vaccines
Diagnostics
CD Markers
🦠 Innate Immunity
Innate immunity
the immunity you're born with — fast, non-specific, no memory
Adaptive immunity
the specific, learned immunity — slower to start, but remembers
PAMP
a molecular pattern found on pathogens, not on human cells
DAMP
a signal released from damaged/dying host cells, not a pathogen
PRR
a receptor that recognizes PAMPs/DAMPs and triggers a response
TLR (Toll-like receptor)
a common type of PRR found on innate immune cells
Chemotaxis
cell movement toward a chemical signal
Phagocytosis
a cell engulfing and destroying a pathogen
Opsonization
coating a pathogen so it's easier for phagocytes to grab and eat
Respiratory burst
a rapid release of reactive oxygen species to kill engulfed microbes
Inflammation
the body's protective response to injury or infection — redness, heat, swelling, pain
Acute phase reactant
a blood protein (like CRP) that rises quickly during inflammation
Interferon
a signaling protein that puts nearby cells on "antiviral alert"
Cytokine
a chemical messenger immune cells use to talk to each other
Chemokine
a cytokine whose job is specifically to attract cells to a location
🧫 Cells & Organs
Neutrophil
the first-responder white cell, mainly eats bacteria
Macrophage
a big "eater" cell that also presents antigen to T cells
Dendritic cell
the professional antigen-presenting cell, links innate and adaptive immunity
NK cell (natural killer)
kills infected or tumor cells without needing prior exposure
Mast cell
tissue-resident cell that releases histamine, key in allergy
Eosinophil
fights parasites, also involved in allergic reactions
Basophil
the blood-circulating cousin of the mast cell
B cell (B lymphocyte)
the lymphocyte that matures into antibody-making plasma cells
T cell (T lymphocyte)
the lymphocyte that coordinates or directly kills infected cells
Helper T cell (CD4+)
coordinates the immune response by helping B cells and other T cells
Cytotoxic T cell (CD8+)
directly kills cells infected with intracellular pathogens
Plasma cell
a fully differentiated B cell whose job is to pump out antibodies
Memory cell
a long-lived cell left over after infection, gives a faster 2nd response
Antigen-presenting cell (APC)
a cell that displays antigen fragments on its surface for T cells to see
Thymus
where T cells mature and are educated to ignore self
Bone marrow
where all blood and immune cells originate, and where B cells mature
Germinal center
a site inside a lymph node where B cells mature and improve their antibodies
🎯 Adaptive Immunity
Antigen
any molecule that can trigger an immune response
Epitope
the exact small part of an antigen that a receptor actually binds
MHC class I
found on almost all nucleated cells, shows antigen to CD8+ T cells
MHC class II
found on antigen-presenting cells, shows antigen to CD4+ T cells
Clonal selection
only the lymphocyte that matches the antigen gets picked to multiply
Clonal expansion
the rapid multiplying of that selected lymphocyte
Affinity maturation
antibodies get progressively better at binding their target over time
Class switching
an antibody changes its isotype (e.g. IgM to IgG) without changing its target
Humoral immunity
the antibody-based arm of adaptive immunity (B cells)
Cell-mediated immunity
the T-cell based arm of adaptive immunity
Central tolerance
removing self-reactive lymphocytes in the thymus/bone marrow before release
Peripheral tolerance
backup mechanisms that keep self-reactive cells in check outside the thymus/marrow
Anergy
a lymphocyte that's alive but functionally switched off
Immunologic memory
the ability to respond faster and stronger on a second exposure
🧬 Antibodies & Complement
Antibody (immunoglobulin)
a Y-shaped protein made by plasma cells that binds a specific antigen
IgM
the first antibody made in a new response, good at activating complement
IgG
the main long-term antibody, crosses the placenta
IgA
the main antibody in secretions — saliva, tears, breast milk, mucus
IgE
the antibody behind allergies and anti-parasite responses
IgD
mostly found on the surface of naive B cells, function still unclear
Complement system
a cascade of blood proteins that helps kill pathogens and drive inflammation
Membrane attack complex
the final complement product that punches a hole in a pathogen's membrane
Agglutination
antibodies clump particles/cells together by cross-linking antigens
Neutralization
antibody blocks a toxin or virus from binding/entering a cell
⚡ Hypersensitivity & Autoimmunity
Hypersensitivity
an exaggerated immune response that ends up damaging the body
Type I hypersensitivity
IgE-mediated, immediate — classic allergy/anaphylaxis
Type II hypersensitivity
antibody binds directly to an antigen on a cell surface
Type III hypersensitivity
antibody-antigen immune complexes deposit in tissue and cause damage
Type IV hypersensitivity
delayed, T-cell mediated, no antibody involved
Autoimmunity
the immune system mistakenly attacks the body's own tissue
Self-tolerance
the immune system's normal ability to leave self antigens alone
🛑 Immunodeficiency
Primary immunodeficiency
a genetic/inborn defect in the immune system
Secondary immunodeficiency
an acquired immune defect, e.g. from HIV, malnutrition, or drugs
SCID
severe combined immunodeficiency — both T and B cell arms fail
Opportunistic infection
an infection that only takes hold because the immune system is weakened
🔄 Transplant & Vaccines
HLA
human leukocyte antigen — the human version of MHC, used for matching
Graft rejection
the recipient's immune system attacks the transplanted tissue
GVHD
graft-versus-host disease — transplanted immune cells attack the recipient
Active immunity
immunity your own immune system builds (infection or vaccine)
Passive immunity
immunity received ready-made from an outside source (antibodies, placenta)
Vaccine
a substance that trains the immune system to recognize a future threat
🔬 Diagnostics
ELISA
a test that uses antibodies and a color/enzyme signal to detect a target
Western blot
a test that separates proteins by size, then detects them with antibodies
Serology
testing blood serum for antibodies or antigens
🏷️ CD Markers (Cluster of Differentiation)
CD (Cluster of Differentiation)
a numbered "name tag" system for surface proteins used to identify and sort immune cells
CD3
on all T cells, part of the TCR complex that sends the activation signal inward
CD4
on helper T cells, binds MHC class II, also the receptor HIV uses to enter cells
CD8
on cytotoxic T cells, binds MHC class I
CD19
on B cells, general B-cell marker used to identify them from birth to plasma cell
CD20
on B cells, the target of rituximab therapy
CD28
on T cells, the main costimulatory receptor needed alongside signal 1 for full activation
CD40
on B cells, pairs with CD40L on helper T cells to trigger class switching
CD40L (CD154)
on activated helper T cells, missing in hyper-IgM syndrome
CD56
a classic marker used to identify NK cells
CD16
on NK cells, an Fc receptor that lets them recognize antibody-coated targets (ADCC)
CD14
on monocytes/macrophages, helps recognize bacterial LPS alongside TLR4
CD25
part of the IL-2 receptor, marks activated T cells and regulatory T cells
CD34
on hematopoietic stem cells, used to identify and isolate them
CD80/CD86 (B7)
on APCs, the costimulatory partner that binds CD28 (signal 2)
CD95 (Fas)
a death receptor that triggers apoptosis when bound by Fas ligand
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