What is antibody?
Antibody, Y shaped protein complex belonging to immunoglobulin(Ig) superfamily, exists in many kinds of organisms, especially higher animals. Specific antibody can bind specifically to particular exogenous pathogen like virus or bacteria to either directly neutralize the pathogens by blocking its active sites or facilitate the elimination of pathogens by other immune cells, such as macrophages. Antibody is produced by B lymphocytes (plasma cells). B cells initially express antibodies on the cell membrane as B cell receptors (BCR). When BCRs are coupled with desired pathogens, B cells are differentiated into plasma cells, producing a myriad of antibodies. Part of plasma cells transform into memory cells which respond relatively fast to the same substance upon later infection.
Most antibodies are Y shaped and comprises four subunits, two heavy chains and two light chains. Each light chain and a moiety of each heavy chain pairs together via one disulfide bond to form the upper branch of ‘Y’, also named as Fab (antigen-binding fragment). The rest part of two heavy chain form the root of ‘Y’ via two disulfide bonds, also named Fc. Collectively, one antibody has two Fabs and one Fc. The top part (N termini) of Fab are the variable domains of light chain (VL) and heavy chain (VH) with the antigen-binding site on the tip, and the rest part, involving some part of Fab and the whole part of Fc, is the more constant domain (CL and CH). Accordingly, Fab decides the specificity and affinity of antibody to antigen while Fc determines the locations of antibody.