Pathogens are infectious agents that cause illness or disease to its host. Pathogens are categorized into 4 main groups:
(A) Viruses: Genetic material (RNA or DNA) inside a protective protein capsid. (B) Bacteria: release toxins or break down surrounding tissue, (C) Parasites: Can survive inside or outside of human cells and (D) Prions: Proteinaceous infection particles, which are abnormallystructured host proteins.
The Lymphatic System
Lymphoid organs: (A) spleen - is made up of masses of lymphoid tissue which are located around terminal branches of the circulation, (B) thymus - is made up of 2 lateral lobes, which are enclosed in a capsule. Each lateral lobe is made up of many smaller lobules and (C) lymph nodes - Lymph nodes are located throughout the body and serve as filters for tissue fluid.
Lymph fluid: is made up of: (1) Fluid from the intestines containing proteins and fats, (2) A few red blood cells and (3) Many lymphocytes. Lymph (originally tissue fluid) is collected in the lymphatic vessels and ultimately transported back into the systemic circulation by the pressure in the tissue, skeletal muscle activity and a series of one-way valves.
The Immune System:
Humoral immunity: Primary response: The very first time the lymphocytes meet a particular antigen, plasma cells produce antibodies to kill the pathogen. Memory B cells remember how to kill the antigen. Secondary response: Exposure to the same antigen later triggers a stronger immune response, because the system is already prepared.
Cellular Immunity: T cells kill infected cells in the cell-mediated response. Once inside cells, pathogens are harder to detect. Cell-mediated immunity recognizes and kills the body’s own infected cells.
B-cells: Develop in the bone marrow and become antibody-producing plasma cells. Bind antigens to surface-bound antibodies.
T-cells: Develop in the thymus; differentiate into T-helper cells or T-cytotoxic cells.Antibodies: Antibodies are soluble proteins that are bound to the surface of cells, as well as unbound in the circulation. There are 5 types (isotypes) of antibodies: IgA: protects mucosal surfaces, IgD: B-Cell antigen receptor, IgE: involved in allergy, IgG: majority of antibody-based immunity and IgM: key to B-Cell immunity.