Learning Objective

Differentiate immune cell types based on their characteristic cell surface markers, and understand how these markers relate to immune function, signaling, antigen presentation, and pathogen recognition.

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T Cells

• TCR – Binds antigen–MHC complex
• CD3 – Signal transduction complex associated with TCR
• CD28 – Binds B7 (CD80/86) on antigen-presenting cells for costimulation

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Helper T Cells (Th)

• CD4
• CD40L – Activates B cells and macrophages
• CXCR4 / CCR5 – HIV coreceptors

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Cytotoxic T Cells (Tc)

• CD8

Regulatory T Cells (Treg)

• CD4
• CD25 (IL-2 receptor α-chain)
• Often express FOXP3 (transcription factor; not a surface protein)

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B Cells

• Immunoglobulin (surface IgM/IgD) – Antigen binding
• CD19, CD20, CD21
  • CD21 is the receptor for EBV
  • Mnemonic: “Must be 21 to drink at a Bar(r)”
• CD40 – Required for class switching
• MHC II
• B7 (CD80/86) – Costimulatory molecule

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Natural Killer (NK) Cells

• CD16 – Binds the Fc region of IgG (ADCC)
• CD56 – Classic NK cell marker

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Macrophages

• CD14 – Receptor for PAMPs (eg, LPS)
• CD40
• CCR5
• MHC II
• B7 (CD80/86)
• Fc and C3b receptors – Enhance opsonization and phagocytosis

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Hematopoietic Stem Cells

• CD34

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Activity