Immune Defense Army
Humans are blessed with an immune system which can recognize foreign invaders and fight them like an army in a battle. The army constitutes of various types of White Blood Cells (WBCs), each contributing to the defense mechanism with their own special set of tasks. Antigen Presenting Cells (APC’s), a type of WBCs, are the front-runners or leaders of the army.
Each organism, whether virus or bacterium, carries surface tags which are also called antigens. A vaccine contains these specific antigens, derived from the pathogen in attenuated, inactivated or partial form. These vaccine antigens are able to elicit or induce the immune response without causing any disease.
Identify the Foreign Invader
When vaccine is given in the form of an injection or oral drops, this antigen concoction enters the human body and encounters the circulating APC’s . The APC’s have a nature to gulp anything which they recognize as foreign. They consume the vaccine components and display only the antigens in even simpler form.
Deployment of Army
APC’s then move to the lymph nodes, which are like base camps, placed throughout the body. Here, lymphocytes, members of the special task force, are positioned. Lymphocytes are of two types, T cells and B cells. They read alarming signals in form of vaccine antigens presented by APC’s and start their activity. This is how the immune response is induced.
Bombing the enemy occupied Territory
T cells have two cadres, the offensive or killer ones and the defensive or helper ones. T helper cells associate with these APC’s after decoding the antigenic signals. They are very important cadres as they perform multiple tasks at the base, like calling phagocytes to eat up the microbes, activating T killer cells and stimulating B cells. T killer cells don’t attack the enemy or pathogen directly, but use the chemical weapons to eliminate the pathogen occupied cells. They identify these diseased cells using the information displayed in the form of antigenic signals. The task done by T cells is called cell mediated immune response.
B cells, upon receiving the signal from T helper cells, evolve into big factories called plasma cells. These plasma cells generate chemical weapons called antibodies in large amounts. These antibodies can directly attach to and disable the specific pathogens in the blood which are yet to infect any host cell. The antibodies call upon the phagocytes and other big eaters to consume the pathogens. The task done by B cells is called humoral immune response.
Most of the vaccines act by stimulating antibody response alone since, when generated in mammoth amount, antibodies are sufficient weapons to fight the foreign invaders in one on one fight. Battle turns into the favor of immune system when T cells and antibodies begin to outnumber the foreign antigens, finally leading to their disappearance from the body.
Winners return to base and guard against future invasions
After the battle is won, the victorious B cells and T cells get promoted to memory cells. Memory B cells are packed with an armour to quickly transform into plasma cells and make larger amount of antibodies against specific pathogen when needed. Memory T cells are capable of dividing and growing into a full-fledged battalion of pathogen fighters. In case of reinfection with the pathogen for which the vaccine has already been given to the person, the immune system, already brimming with experienced memory cells, quickly recognizes and clears the infection.
Image credit: http://www.historyofvaccines.org/content/how-vaccines-work