Question

Make a table with two columns to compare and contrast the ways in which T cells and
B cells recognise antigen, the antigens that are recognised by the two types of cell, and
the ways in which the cells become activated. Aim to make at least five distinct
comparisons.

Answer 1

The major players of immune responses are the T cells and B cells. Both these cells are produced in the bone marrow. But the maturation of T cells take place in thymus.

The two pathways by which immune responses are activated include :

1. Cell mediated immune response : This immune response is initiated by T cells and variety of other immune cells play a major role in cell mediated immune response.

T cells can be subdivided into TH cell (T helper cell) or CD4 cell and TC cell (T cytotoxic cell) or CD 8 cell.

2.Humoral immune response : Humoral responses are mediated by B cells and result in the secretion of antibodies from plasma cell, and memory cells which retains the immunological memory.

The activation of B cells occur either by T cell (T cell mediated pathway) or B cell activation occurs without T cells (T cell independent pathway)

T Cells	B Cells
T cells play a major role in cell mediated immunity	B cells are involved in humoral immune response and secrete antibodies.
T cells are activated by antigen processed and presented by MHC (Major Histocompatibility Complex) on the surface of Antigen Presenting Cell (APCs)	T cell independent antigens include foreign polysaccharides and unmethylated CpG DNA.
CD4 cells are activated by presentation of antigenic peptide by MHC class II on the surface of APCs. CD 8 cells are activated by the antigenic peptides on the surface of APCs presented by MHC class I molecule. T cells bind to the antigenic peptide on APC via TCR ( T cell receptor).	T cell dependent antigen include foreign proteins.
Following activation, TH cells (CD4 cells) divide rapidly and secrete cytokines which regulate immune response. TH cells can activate either B cell for antibody mediated response or other downstream immune cells for cell mediated response.	B cells are activated when they encounter an antigen while circulating through the secondary lymphoid organ and binds to the antigen via BCR (B cell receptor).
TC cells also called cytotoxic lymphocytes (CTLs) or CD 8 cells are activated by virus infected cells and tumor cells, also involved in graft rejection. Tc cells secrete cytotoxins which destroy the infected or abnormal cells.	Activated B cells mediate the production of short lived plasma blasts for immediate response and long-lived plasma cells and memory cells for long term protection and retain immunological memory. The plasma cells undergo immunoglobulin class switching and secrete antibodies. Early antibodies secreted by plasmablasts are of IgM type.