Antigenic drift is especially common for the HA protein, in which just a few amino acid changes in the head region can constitute antigenic drift. The result is the production of novel strains that can evade pre-existing antibody-mediated immunity. Antigenic drift occurs in all influenza species but is slower in B than A and slowest in C and D. Antigenic drift is a major cause of seasonal influenza, and requires that flu vaccines be updated annually. HA is the main component of inactivated vaccines, so surveillance monitors antigenic drift of this antigen among circulating strains. Antigenic evolution of influenza viruses of humans appears to be faster than influenza viruses in swine and equines. In wild birds, within-subtype antigenic variation appears to be limited but has been observed in poultry.Antigenic shift is a sudden, drastic change in an influenza viruss antigen, usually HA. During antigenic shift, antigenically different strains that infect the same cell can reassort genome segments with each other, producing hybrid progeny. Since all influenza viruses have segmented genomes, all are capable of reassortment. Antigenic shift, however, only occurs among influenza viruses of the same genus and most commonly occurs among IAVs. In particular, reassortment is very common in AIVs, creating a large diversity of influenza viruses in birds, but is uncommon in human, equine, and canine lineages. Pigs, bats, and quails have receptors for both mammalian and avian IAVs, so they are potential "mixing vessels" for reassortment.
Step-siblings are children born of two different families who have been joined by marriage, A male step-sibling is a stepbrother and a female is a stepsister. The step-siblings relationship is connected through law and is not a blood relation. Step-siblings are sometimes abbreviated informally as stepsibs. Culture
In many fairy tales, the central character has a stepmother and the step-siblings serve as an extension of their mother. Cinderella and Mother Hulda features wicked stepsisters who take after their parents. The story Kate Crackernuts serves as a counterexample where the daughter of the evil stepparent is a loving stepsisters.Many romance novels feature heroes who are the stepbrother of the heroine. The step-relationship generally stems from a marriage when the hero and heroine are at least in their adolescence.Some family films and television sitcoms feature a blended nuclear family including siblings as the center premise. In many cases, the step-family is large and full of children causing situations such as sibling rivalry, rooming, falling in love, and getting along amongst the children as popular plot-lines. This premise gained traction with the 1968 films Yours, Mine and Ours and With Six You Get Eggroll and the 1969 launch of the television sitcom The Brady Bunch. Some contemporary family sitcoms have made the blended family sitcom more popular with the TGIF show Step by Step bringing about other shows such as Aliens in the Family, Life with Derek, Drake & Josh, and the short-lived NBC family sitcom Something So Right.