Antigenic variation

• Antigens on the surface of pathogens activate the primary response. When you're infected a second time with the same pathogen which has the same antigens on its surface they activate the secondary response and you don't get ill
• However some pathogens can change their surface antigens - antigenic variation (different antigens are formed due to changes in the genes of a pathogen)
• This means that when you are infected for a second time, the memory cells produced from the first infection will not recognise the different antigens - the immune system has to start from scratch and carry out a primary response again to these new antigens. This primary response takes time to get rid of the infection - get ill again
• Antigenic variation makes it very difficult to develop vaccines against some pathogens for the same reason. Examples of pathogens that show antigenic variation include HIV and the influenza virus
• The influenza vaccine changes every year - antigens on the surface of influenza virus change regularly, forming new strains
• Memory cells produced from vaccination with one strain will not recognise other strains with different antigens - strains are immunologically distinct.