Pathogens and the Immune System

• Disease-causing organisms like viruses, bacteria, fungi, and protists are known as pathogens.
• Archaea are not known to cause diseases in humans.
• Careful observation led to breakthroughs in infectious disease control during 19th-century epidemics.

Primary Defense

• The skin acts as a physical and chemical barrier to pathogens.

Blood Clotting

• Platelets release clotting factors, initiating a cascade that rapidly converts fibrinogen to fibrin.
• Thrombin facilitates this conversion, and erythrocytes are trapped to form a clot, sealing cuts in the skin.

Immune System

• The innate immune system responds to broad categories of pathogens without changing during an organism’s life.
• The adaptive immune system responds specifically to particular pathogens, building up a memory for a more effective immune response upon re-exposure.
• Phagocytes are components of the innate immune system.

Phagocytes

• Phagocytes move from the blood to sites of infection.
• Phagocytes recognize pathogens, engulf them through endocytosis, and digest them using enzymes from lysosomes.

Lymphocytes

• Lymphocytes circulate in the blood and are contained in lymph nodes.
• Each individual has a large number of B-lymphocytes that make a specific type of antibody.

Antigens

• Antigens are recognition molecules that trigger antibody production.
• Most antigens are glycoproteins or other proteins located on the outer surfaces of pathogens.
• Antigens on the surface of erythrocytes can stimulate antibody production if transfused into a person with a different blood group.

B-Lymphocytes

• Antigen-specific B-cells and helper T-cells exist.
• B-cells produce antibodies and become memory cells only after activation.
• Activation requires direct interaction with the specific antigen and contact with a helper T-cell also activated by the same antigen.

Antibody Production

• Activated B-cells divide by mitosis to produce large numbers of plasma B-cells.
• Plasma B-cells produce the same type of antibody.

Immunity

• Immunity is the ability to eliminate an infectious disease from the body.
• Memory cells, which are long-term surviving lymphocytes, are responsible for making specific antibodies to fight infection.

HIV Transmission

• HIV (human immunodeficiency virus) is transmitted through body fluids.

HIV and AIDS

• HIV infects and kills certain types of lymphocytes.
• A reduction in lymphocytes limits the ability to produce antibodies and fight opportunistic infections, leading to AIDS.

Antibiotics

• Antibiotics block processes occurring in bacteria but not in eukaryotic cells.
• Antibiotics fail to control infection with viruses.

Antibiotic Resistance

• Pathogenic bacteria can evolve resistance to several antibiotics.
• Careful antibiotic use is necessary to slow the emergence of multi-resistant bacteria.
• Searching chemical libraries is yielding new antibiotics.

Zoonoses

• Zoonoses are infectious diseases that can transfer from other species to humans.
• Examples of zoonoses include tuberculosis, rabies, Japanese encephalitis, and COVID-19. COVID-19 recently transferred from another species and has had profound consequences for humans.

Vaccines and Immunization

• Vaccines contain antigens or nucleic acids (DNA or RNA) that code for antigens.
• Vaccines stimulate the development of immunity to a specific pathogen without causing the disease.

Herd Immunity

• In herd immunity, a sufficient percentage of a population is immune to a disease, greatly impeding transmission.
• Members of a population are interdependent in building herd immunity.

Research and Evaluation

• Scientists publish research for evaluation by other scientists.
• The media may report on research while evaluation is still happening.
• Vaccines are rigorously tested, and the risks of side effects are minimal but not nil.