U01.02.024 Cell surface proteins

Understanding cell surface proteins is crucial for immunology and cell biology. Below is a detailed summary of the important cell surface proteins associated with different immune cells, their functions, and some key markers you should be familiar with for the USMLE Step 1.

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Key T Cell Surface Proteins

Cell Type	Protein(s)	Function
T Cells	TCR (T cell receptor)	Binds to antigen-MHC complex for antigen recognition
	CD3	Associated with TCR for signal transduction
	CD28	Binds to B7 on antigen-presenting cells (APCs), providing co-stimulation

Helper T Cells

Cell Type	Protein(s)	Function
Helper T Cells	CD4	Co-receptor for MHC II, enhances antigen recognition
	CD40L	Interacts with CD40 on APCs for immune activation
	CXCR4, CCR5	Co-receptors for HIV infection

Cytotoxic T Cells

Cell Type	Protein(s)	Function
Cytotoxic T Cells	CD8	Co-receptor for MHC I, involved in the killing of infected cells

Regulatory T Cells

Cell Type	Protein(s)	Function
Regulatory T Cells	CD4, CD25	Involved in immune regulation and suppression

B Cells

Cell Type	Protein(s)	Function
B Cells	Ig (Immunoglobulin)	Binds to antigen for immune response
	CD19, CD20, CD21	Receptor for Epstein-Barr virus (EBV), involved in B cell signaling
	CD40	Co-stimulatory molecule for activation by helper T cells
	MHC II, B7	Involved in antigen presentation and co-stimulation

Macrophages

Cell Type	Protein(s)	Function
Macrophages	CD14	Receptor for pathogen-associated molecular patterns (PAMPs), e.g., LPS
	CD40	Interaction with CD40L from helper T cells for immune activation
	CCR5	Co-receptor for HIV and other infections
	MHC II, B7	Involved in antigen presentation and co-stimulation
	Fc and C3b Receptors	Enhance phagocytosis by binding IgG and complement proteins

NK Cells

Cell Type	Protein(s)	Function
NK Cells	CD16	Binds Fc of IgG, involved in antibody-dependent cell-mediated cytotoxicity
	CD56	Marker for NK cells, involved in recognizing and killing infected cells

Hematopoietic Stem Cells

Cell Type	Protein(s)	Function
Hematopoietic Stem Cells	CD34	Marker for hematopoietic stem cells, used in identification and isolation

Key Points to Remember:

TCR (T cell receptor) binds the antigen-MHC complex, crucial for T cell activation.
CD28 on T cells binds to B7 on APCs, providing co-stimulation for full activation.
CD4 and CD8 are co-receptors that enhance antigen recognition via MHC II and I, respectively.
Helper T cells express CD4, CD40L, and co-receptors for HIV (CXCR4, CCR5).
Regulatory T cells help suppress immune responses to maintain tolerance, marked by CD4 and CD25.
B cells use Ig to bind antigens, and CD19, CD20, CD21 are key for signaling and EBV infection.
Macrophages have receptors like CD14, CD40, and CCR5, important for pathogen recognition and immune activation.
NK cells are identified by CD56 and are important for killing virus-infected or tumor cells.

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