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The immune system has two main branches: the innate immune response and the adaptive immune response.
The innate immune response includes cells and mechanisms that are able to react to pathogens from birth. The adaptive immune system can learn and remember new pathogens and then protect from them long-term.
The innate immune system is composed of:
1. physical barriers such as mucous membranes and skin
2. chemical barriers such as lysozyme and stomach acid.
3. Complement is a group of small circulating chemicals that, when triggered, form a "membrane attack complex" that can punch holes in pathogen cell membranes.
The leukocytes that are involved in this response include:
*Granulocytes - neutrophils, basophils, eosinophils
*Agranulocytes - macrophages and dendritic cells, which can be phagocytic and present antigens from invaders to the adaptive immune response cells known as helper T cells.
*Lymphocytes - Natural killer cells (NK cells) can cause apoptosis of tumor cells and viral-infected cells.
The adaptive immune response includes T and B lymphocytes. These cells must be activated to a specific pathogen before they can respond.
Helper T cells stimulate cytotoxic T cells to directly destroy pathogens. Helper T cells stimulate B lymphocytes to enlarge into plasma cells and produce antibodies.
These lymphocytes all reproduce rapidly and form memory cells that remain circulating and passing through lymph organs long-term to patrol for the presence of the specific pathogen in the future.
All leukocytes are formed in the bone marrow in a process known as hematopoiesis.
