Considering the amazing quantities and diversity of pathogens that exist in the environment, vertebrates had to develop a complex defense system, or they would have perished. In humans, as in most animals, there are multiple lines of defense against potential pathogens. The integumentary membranes (skin and mucous membranes) provide a physical barrier to pathogens. If these are compromised, the inflammatory response is stimulated, during which neutrophils and macrophages phagocytize foreign invaders nonspecifically.
If an infection becomes systemic, the immune response is the last resort. Lymphocytes, specialized white blood cells that mainly reside in the lymph nodes, specifically attack pathogens in the blood and lymph. B lymphocytes are responsible for antibody-mediated immunity, which involves the reaction of protein antibody molecules with antigenic markers on the pathogen. T lymphocytes are responsible for cell-mediated immunity, during which antigens are presented to T cells by antigen presenting cells of the human body, often macrophages or virally infected cells.
Several diseases can cause the immune system to become weakened or almost completely destroyed. AIDS (Acquired Immune Deficiency Syndrome) is a disease caused by the HIV virus, which attacks helper T cells that normally produce chemicals (interleukins) that stimulate the entire immune system. While functional cytotoxic T cells and plasma B cells can be created, their general activity is low due to lack of interleukin stimulation. Type I diabetes mellitus is an example of an autoimmune disease, in which lymphocytes attack normal body cells, in this case the insulin producing cells of the pancreas. Perhaps the most devastating immune disease is called severe combined immune deficiency (SCID); individuals with this luckily rare condition have virtually no functioning lymphocytes, and often must live in vinyl bubbles to completely cut them off from any contact with pathogens.
Which physical barrier would be most susceptible to infection?