What is one specific component of the first line of defense

Solution

The first line of defence includes physical and chemical barriers that are always ready and prepared to defend the body from infection.These include skin,tears,mucus,cilia,stomach acid,urine flow,friendly bacteria and white blood cells called neutrophils.

Skin consists of two layers:
epidermis -outer thinner portion made of tightly packed cells
dermis -inner thicker portion;Gives skin strength         
Infection can develop when the epithelial surface is broken
Mucous membranes
Mucous membranes line the gastrointestinal,respiratory and genitourinary tracts.Epithelial layer secrets the mucus that maintains the surface of the membrane always moist.Mucous membranes are more susceptible to infections than skin.

Sebum -oily substance produced by skin; contains unsaturated fatty acids.It inhibit the growth of certain pathogenic bacteria.
Gastric juice -mixture of hydrochloric acid,enzymes,and mucus;kills most of bacteria except Clostridium botulinum and Staphylococcus aureus
Lysozyme in saliva and tears- enzyme that hydrolyzes the peptodoglycan.

some other chemical defense mechanisms that the body has
pH (skin and stomach),beta lysins(in the blood, lyse bacterial cells),sebaceous gland secreations(fatty acids lower pH),lactic acid secreted from many skin normal flora organisms,lactoferrin and transferrin.