Surface Membrane Barriers

The epidermis, the outer layer of skin, consists of densely packed keratinized cells that form a robust physical barrier preventing microbes from penetrating the body. The periodic shedding of epidermal cells helps eliminate microbes from the skin surface. However, injuries such as cuts or burns can compromise this barrier, allowing pathogens to invade tissues or enter the bloodstream.

Sebaceous glands in the skin secrete sebum, an oily substance that forms a protective film over the skin surface. Sebum possesses antimicrobial properties, including unsaturated fatty acids that inhibit the growth of certain pathogenic bacteria and fungi. The skin's acidity, partly due to fatty acids and lactic acid in sebum, further helps flush microbes from the skin surface.

Mucous membranes lining body cavities produce mucus, a viscous fluid that entraps microbes and foreign particles. In the nasal passages, hair filters airborne microbes, while cilia in the upper respiratory tract propel trapped dust and microbes toward the throat for expulsion through coughing, sneezing, or swallowing.

Tears, saliva, sweat, and nasal secretions contain lysozyme, an enzyme that breaks down bacterial cell walls. Gastric juice combines hydrochloric acid, enzymes, and mucus with potent acidity that destroys many bacteria and bacterial toxins. The low pH of sweat and vaginal secretions creates a hostile environment for bacterial growth.

Both mucous membranes and skin secrete antimicrobial peptides like defensins that inhibit microbial growth. These peptides work alongside physical barriers and other chemical defenses to prevent pathogen colonization. Together with enzymes like lysozyme and acidic secretions, defensins form part of the body's initial defense against pathogens.

Surface barriers such as skin and mucous membranes provide both physical and chemical protection against pathogens, preventing microbial entry and establishment. These barriers are essential for maintaining host integrity and represent the initial defense before cells of the innate immune response become involved in protecting the body.

While bacteria rarely infiltrate the intact surface of healthy epidermis, injuries such as cuts or burns compromise this barrier, allowing pathogens to invade underlying tissues or circulate in the bloodstream. Once the physical barrier is breached, microbes can bypass the protective mechanisms of surface membranes and establish infection.