Question

Draw a circulating IgG antibody and a TCR on the membrane of a T cell. Identify specifically the constant and variable Ig domains on each molecule, identify each of the protein chains, illustrate specifically where the antigen interaction takes place and for the circulating IgG, identify and name the region which is responsible for the antibody effector functions.

Answer 1

Immunoglobulin or Ig is a type of antibody. Different Ig are there in the serum among them IgG is the most abundant in circulation. The structure of IgG mainly consists of two identical heavy chain and two identical light chain polypeptides. The light chains are attached to heavy chains and the two heavy chains are attached together by disulfide bonds. Both the heavy and light chain consists of variable and constant domains. The constant domain will be same for all the IgG antibodies whereas the variable domains will be different. The variable domain provides the specificity to the antibody to bind with a particular antigen. The variable domain of heavy and light chain constitutes the antigen binding site. The FC region is responsible for binding it to the cell receptor. The structure of circulating IgG drawn is given below:

Once the IgG gets activated by binding to the antigen, it will bind with the receptors present in the cell surface called Fc receptors and to some complement proteins. The effector region of IgG is the Fc region which binds with the effector cells and promote the destruction of pathogens or toxins.

The structure of membrane bound T cell receptor is given below:

T-cell receptors contain two polypeptide chains, alpha and beta. A portion of the molecule is found outside the cell, portion anchoring the cell membrane of t-cell and has a cytoplasmic tail. The TCR has a single antigen binding site, whereas the antibody has two antigen binding site. The variable regions of alpha and beta chains forms the antigen binding site of the TCR.