The Immune System

The immune system has the major function of defending the body from invaders such as pathogenic bacteria and viruses which can lead to diseases, sicknesses, and death. The functional unit of this system are white blood cells called lymphocytes and antibodies that have the ability to attach to microbes and mark them for death and the lymph cells kill them. There are three major lines of defense.

The first line of defense is the external defenses such as the skin, mucous membranes, and secreted substances and wastes. The skin covers the body, stopping any entering pathogens from entering organ systems. All major organs are covered by mucous to protect them from pathogens from entering and harming the systems by trapping them such as dust and allergies or molecules in the nose from hurting the lungs. Tears, saliva, urine, and feces secrete out of the body removing toxins, poisons, and pathogens from within through pores. The secretions contain antimicrobial proteins such as lysosomes that can digest bacterial cell walls, therefore suspending bacterial internal substances and killing the pathogenic cells. These are present in saliva, tears, and mucous.


The second line of defense consists of internal defenses for microbes that enter the body and defeat external defenses. This along with the first line, is an innate immunity. These act without any specializations for specific microbes, and all depend on phagocytes, the ingestion of invading microorganisms by white blood cells called phagocytes. Phagocytes produce antimicrobial proteins that initiate inflammation and limit the spread of pathogens. They attach to the invaders through surface receptors, and then engulf the microbe. Non-phagocytic white blood cells are called natural killers. These cells detect abnormal receptors on virus infected and cancer cells, releases chemicals that cause apoptosis, or “cell suicide” and are effective. The most abundant phagocytic white blood cells (leukocytes) are neutrophils which are attracted to infected tissues, but they self destruct after phagocytosis. Macrophages are large leukocytes that develop from monocytes, that migrate throughout the body.


Eosinophils attack multi-cellular parasites like blood flukes, and dendritic cells ingest microbes and stimulate acquired immunity. Antimicrobial proteins either attack microbes or halt their reproduction. 30 serum proteins make up the complement system which is triggered by surface proteins on microbes that cause lysis where invaders burst. Interferon A and interferon B fight viruses and are secreted by virus infected body cells and induce adjacent uninfected cells. Inflammation is when a damaged tissue swells up and releases chemical signals. Histamine is a signal stored in most cells in connective tissues. These release histamine triggering dilation and permeability of capillaries. Blood flow increases which causes red spots and allows phagocytes to enter as well.

The third line is called acquired immunity and consists of cell mediated responses (cytotoxic lymphs) and Humoral responses (antibodies). Mutations allow specific recognizations of microbes for specific responses. Humoral responses involve the activation and clonal selection of B cells which causes secretion of antibodies in the blood. Cell mediated responses involves activation and lonal selection of cytotoxic T cells which kill infecting target cells. These use B and T cells that are activated by the helper T cells that respond to peptide antigens on antigen presenting cells. B cells are covered with receptors and they release immunoglobins. T cells have receptors that hunt tagged and invading cells.

T Cell

Two types of responses, regarding time and exposure, exist in the immune system. Primary response occurs to antigen A and produces antibodies to A and has no previous exposure so it takes longer to peak, around 10-17 days. Secondary response to Antigen A produces A antibodies and is faster, around 2-7 days, and is more effective because the body has been exposed to the invader before.

Written By: Neil 6/17/2019

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