Difference between B Cells and T Cells

T cells and B cells are white blood cells that are important cells for adaptive immunity. Like all blood cells, they are made in the bone marrow. While B-cells mature in the bone marrow, T-cells travel through the bloodstream to the thymus (a small organ between the lungs and behind the sternum) and mature there. Broadly speaking T cells can be divided into two different types, ‘killer T-cells’ and ‘helper T-cells’.

Regulatory T cells (also called Tregs) are another types of T cells. Tregs have a role in regulating or suppressing other cells in the immune system. 2018 Nobel prize in Physiology and Medicine is related to negative immune regulation. Find more innobelprize.org

B cell and T cell function Killer T cells also known as cytotoxic T cells (CTL) hunt down and destroy cells that are infected with germs or that have become cancerous while helper T cells help B cells to make antibodies. T helper (TH) cells express CD4 molecules and are restricted to recognizing antigens bound to class II MHC molecules, whereas T cytotoxic (TC) cells express CD8 and are restricted to recognizing antigens bound to class I MHC molecules. Cytotoxic T cells kill cells that are infected with viruses or altered self-cell with toxic mediators (perforin and granzymes).

B cells are major cells of humoral (antibody-mediated) immunity, effector B cells (plasma cells) produce antibodies that circulate, capture and destroy antigens.

- Helper T cells and B Cell Interactions (Source: Kubay Immunology) Figure: Helper T cells and B Cell Interactions (Source: Kubay Immunology)

APC and T cell interaction Circulating helper T cells recognize exogenous antigens and produce cytokines. Two major groups of helper T cells are known as Th1 and Th2 cells. Th1 cells predominantly produce interferon-g (IFN-g), which promotes cell-mediated immune mechanisms. Th2 cells produce mostly interleukin-4 (IL-4), which promotes humoral immunity by activating B cells.

When a naïve (virgin) B cell first encounters the antigen that matches its membrane-bound antibody, the binding of the antigen to the antibody causes the cell to divide rapidly (clonal expansion); its progeny differentiates into memory B cells and effector B cells called plasma cells. Plasma cells secrete antibodies which act as major effector molecules of humoral immunity.

Some of the major differences between B Cells and T Cells are tabulated below:

Features	B Cells	T Cells
Maturation	Bone Marrow (Bursal equivalent)	Thymus
Involvement of MHC molecules	None required	Required to display processed antigen
Recognition of Antigen	B Cells can recognize and bind to soluble antigens.	T-cell receptor (TCR) does not recognize the free antigens. T cells can recognize an antigen only when it is associated with self MHC molecule on the surface of a self-cell (either an antigen-presenting cell or altered self cell or on a virus-infected cell and graft).
Chemical nature of antigen	B cells recognize an enormous variety of antigens such as proteins, polysaccharides, and lipids.	T cells recognize protein epitopes displayed together with MHC molecules on self-cells, but some lipids and glycolipids are presented on MHC-like molecules.
Interaction with antigen	Involves binary complex of membrane Immunoglobulin and Antigen	Involves ternary complex of the T-cell receptor, antigen, and MHC molecule
Epitope properties	Accessible, hydrophilic, mobile peptides containing sequential or nonsequential amino acids	Internal linear peptides produced by processing of antigen and bound to MHC molecules
Antigen Specificity	Antigen specificity of each B cell is determined by the membrane-bound antigen-binding receptor (antibody) expressed by the cell.	Antigenic specificity of T Cells is determined by antigen-binding T-cell receptor (TCR) on T Cells. TCR genes are capable of generating on the order of 10^9 unique antigenic specificities.
Peripheral Blood	10-15% of total lymphocytes	70-80% of total lymphocytes
Antigen recognition receptors	Membrane-bound immunoglobulin (IgM or IgD complexed with Igα /Igβ) molecules serve as receptors for antigens.	T cell receptors (TCR) complexed with CD3 (signal-transduction element of the T-cell receptor)
CD markers	CD32/FcγRII (Receptor for Fc region of IgG), CD35 or CR1 Receptor for complement (C3b), and CD40 (Signal transduction)	CD3, CD4 (adhesion molecule that binds to class II MHC molecules; signal transduction), CD8 (adhesion molecule that binds to class I MHC molecules; signal transduction), CD28 (receptor for co-stimulatory B7 molecule on antigen-presenting cells), CD45 (a signal-transduction molecule)

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