Forms The membrane e-bound form of an antibody may be called a surface im unoglobulin(slg)or a membrane B The BCR】 when a specifi duction.o Iicrometer in diameter,on lipd afs that the BCRs 一器 trom competing intluences Antibody-antigen interactions determinants Antibody and antigen interact by spatial complementarity (lock and key).The molecular forces involved in the h n is relativ e rather than absolute.Relatively binding also means it is possible for an antibody to cross-react with different antigens of different relative affinities. Often,once an antibody and antigen bind,they become an immune complex,which functions as a unitary object and can act as an antigen in its own right,being countered by other antibodi es.Similarly,haptens are ct is antigenic Isotypes mammals there are five manmh each heavy cha cally:a(alpha),Y(gamma),(delta),(epsilon),and u(mu).This The antibody isotype of a B cell changes during cell development and activation.Imm never been ex to an antigen,express only the IgM IgM and IgD.The co- B cell ready to respond to antigen.]B cell activation follows engagement of the cell-bound antibody molecule with a n antigen,causing the cell to divide and differentiate into an antibody-producing cell called a plasma cell. In th ted form,the B c rther than a the ion of ant IgG,that have defined roles in the immune system