Immune system of the body fight against foreign organisms or objects that gain entry into the body.
When an organism tries to gain entry into the human body it has to breach following lines of defences.

•First line of defense: Physical barriers that viruses, bacteria must cross
Physical barriers are;
–skin covers ~2 m2
–Mucous membranes that line digestive, respiratory, reproductive tracts cover ~400 m2

•Second line of defense: Innate immune system (germline-encoded receptors -- no adaptation to specific pathogens)
–Macrophages (Greek for “big eater”), neutrophils, natural killer (NK) cells
–Cytokines -- hormone-like proteins that mediate inflammation, Complement proteins
Macrophages, neutrophils and natural killer cells are those cells which engulf the foreign organism (bacteria) when they enter into the human body.
Cytokines and complement system help macrophages to perform their function effectively and they secrete some special substances that mediate immune response.

•Third line of defense (vertebrates only): Adaptive immune system (adapts to defend against specific pathogens using variable receptors)
–B cells make antibodies that vary -- can make an antibody specific for any new antigen
–T cells mediate cellular responses using variable receptors (T cell receptors; TCRs)
When a bacterium enters into the body. Body forms antibodies against that bacterium. Next time when that bacterium tries to enter again, these antibodies combine with it and macrophages easily identify such bacterium and immediately engulf it.

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