The first line of defence (or outside defence system) includes physical and chemical barriers that are always ready and prepared to defend the body from infection. These include your skin, tears, mucus, cilia, stomach acid, urine flow, 'friendly' bacteria and white blood cells called neutrophils.
Your skin is a barrier. The top layer of skin is made up of cells that are packed in a tight, overlapping pattern. Lipids that your body produces seal the tiny space between the cells, creating a barrier that's essentially waterproof and pathogens-proof. When pathogens such as bacteria or viruses land on your skin, they simply cannot enter your body.
Your skin also secretes several substances that help kill pathogens. Sweat, for example, does a lot more than keep you cool on a hot day. It contains an enzyme that breaks down bacterial cells. And, your skin's surface is naturally somewhat acidic, which prevents many types of pathogens from growing.
A learning process which results due to an associated between two stimuli, one a natural stimulus and the other an environmental stimulus is known as classical conditioning.
For classical conditioning, there is a neutral stimulus as well as the unconditioned stimulus.
The unconditioned stimulus is the one which triggers a response in the organism automatically that is naturally without any prior learning.
In the given example, the food pellet is the unconditioned stimulus.
