Understanding Infection and Immunity
Infection and Immunity
What is Infection?
Infection is the clinical term for the colonization of a host organism by external species. In clinical use, infection implies that the colonization is harmful, making the organism a pathogen.
What is Immunity?
Immunity describes having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity has both specific and nonspecific components. Nonspecific components act as barriers or eliminate pathogens before they cause disease. Other components adapt to each new disease, generating specific immunity against the pathogen (innate or adaptive).
Antigens and Antibodies
An antigen is a molecule (usually a protein or polysaccharide) that triggers an adaptive immune response by activating lymphocytes. Antibodies (immunoglobulins) are glycoproteins found in vertebrate bodily fluids. They act as receptors on B lymphocytes and help the immune system identify and neutralize foreign materials like bacteria, viruses, or parasites.
Cellular Immune Response
This response, carried out by T cells, targets microorganisms (viruses, fungi, or certain bacteria) inside cells, where they are protected from antibodies.
Types of T Lymphocytes
- Tc (cytotoxic or killer lymphocytes): Destroy foreign and infected cells. They are involved in transplant rejection. They bind to membrane antigens and release enzymes, perforating and destroying the cell membrane.
- TH (helper T lymphocytes): Activate B lymphocytes to become plasma cells that secrete antibodies. They are essential for cytotoxic T lymphocyte and T suppressor responses to antigens. They also enhance macrophage phagocytosis.
- T suppressor cells: Control the final stage of the humoral response, preventing overproduction of B cells and antibodies after antigen removal.
Humoral Immune Response
This response is carried out by B lymphocytes, each programmed to recognize a specific antigen. Virgin lymphocytes haven’t encountered their specific antigen. Binding between the antibody and the antigen triggers B cell differentiation:
- Plasma cells: Produce soluble antibodies that enter the bloodstream. A single plasma cell can produce an estimated 2,000 antibody molecules per second.
- Memory cells: Some T and B lymphocytes differentiate into memory cells after initial antigen contact. These cells remember the antigen, enabling a faster, more intense response to subsequent infections.
Lymphoblasts > B lymphocytes > Plasma cells > Memory plasma cells
Some B lymphocytes (T-dependent) require both antigen binding and T cell collaboration for activation. Humoral immunity defends against infections by extracellular organisms, such as pneumococci in pneumonia or meningococcal meningitis.
Concept of Immune Response
The immune response is how your body recognizes and defends itself against harmful bacteria, viruses, and foreign substances.