The Third Line of Defence

The third line of defence is antibody-mediated immunity. Antibodies in the bloodstream will recognize foreign substances and act to neutralize or destroy them.

Antibodies are "Y" shaped molecules. The amino acid sequence in the tips of the "Y" varies greatly among different antibodies. This variation of amino acids makes the antibody specific for particular antigens. Think of an antibody as a phone number and an antigen as the person you are calling. The 10-digit number is specific to the person you are calling. An antibody is specific to an antigen.

Watch

Watch the video below that explains cell-mediated immunity. The level of detail is higher than what is required, but it will help you visualize the process.

Antibodies contribute to immunity in three main ways:

1. They can prevent pathogens from entering or damaging cells by binding to antigens; they can stimulate the removal of a pathogen by macrophages and other cells by coating the pathogen, and they can trigger direct pathogen destruction by stimulating other immune responses. For example, antibodies will bind to surface antigens on an invading bacterium. This can prevent the bacteria from entering and damaging healthy cells.
2. The antibodies can also attract macrophages to engulf and destroy the bacterium. 
3. Thirdly, the antibody may stimulate a response where granulocytes (basophils, eosinophils, or neutrophils) will release their toxic granules and killer T-cells will release toxic molecules that ultimately results in the destruction of the invading bacteria. Symptoms of this kind of reaction include swelling, fever, pus, or redness and indicate infection. Symptoms of infections, however, indicate that the immune system is working to regain homeostasis.