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110 SECTIon II Immunology ` Immunology—Immune reSponSeS Immunology ` Immunology—Immune reSponSeS

Cell surface proteins
T cells TCR (binds antigen-MHC complex)
CD3 (associated with TCR for signal
transduction)
CD28 (binds B7 on APC)
Helper T cells CD4, CD40L, CXCR4/CCR5 (co-receptors for
HIV)
Cytotoxic T cells CD8
Regulatory T cells CD4, CD25
B cells Ig (binds antigen)
CD19, CD20, CD21 (receptor for Epstein-Barr Must be 21 to drink Beer in a Barr
virus), CD40
MHC II, B7

Macrophages CD14 (receptor for PAMPs, eg, LPS), CD40
CCR5
MHC II, B7 (CD80/86)
Fc and C3b receptors (enhanced phagocytosis)
NK cells CD16 (binds Fc of IgG), CD56 (suggestive
marker for NK)
Hematopoietic stem CD34
cells

Anergy State during which a cell cannot become activated by exposure to its antigen. T and B cells
become anergic when exposed to their antigen without costimulatory signal (signal 2). Another
mechanism of self-tolerance.

Passive vs active immunity
Passive Active
meanS oF acQuISItIon Receiving preformed antibodies Exposure to exogenous antigens
onSet Rapid Slow
duratIon Short span of antibodies (half-life = 3 weeks) Long-lasting protection (memory)
eXampleS IgA in breast milk, maternal IgG crossing Natural infection, vaccines, toxoid
placenta, antitoxin, humanized monoclonal
antibody
noteS After exposure to Tetanus toxin, Botulinum Combined passive and active immunizations
toxin, HBV, Varicella, Rabies virus, or can be given for hepatitis B or rabies exposure
Diphtheria toxin, unvaccinated patients are
given preformed antibodies (passive)—“To Be
Healed Very Rapidly before Dying”

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