Microbiology - The Immune System | Anatomy and Physiology

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Study GuideMicrobiology–The Immune System1.Antigens: The Targets of the Immune SystemOur immune system is designed to recognize and respond toforeign substancesthat enter thebody. These substances are calledantigens, also known asimmunogens.1.1What Are Antigens?Antigens are substances that can trigger an immune response.Most antigens arelarge molecules, such as proteins or complex carbohydrates. However,smallmoleculescan also act as antigens if they first attach to proteins in the body.•These small molecules are calledhaptens.•By themselves, haptens cannot cause an immune response.•When a hapten binds to a body protein, the combined structure (called aconjugate) cantrigger the immune system.So, size matters—but attachment matters even more.1.2How Antigens Start an Immune ResponseFor most immune responses to begin, antigens must betaken up and processed by macrophages(a type of immune cell found in tissues).Here’s what happens:1.A foreign substance enters the body.2.A macrophage engulfs and processes it.3.This processing helps activate other immune cells.However,not every foreign substance causes an immune response. Some substances may bebroken down or destroyed before macrophages can process them, so no immune reaction occurs.

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Study Guide1.3Antigenic Determinants (Epitopes)Antigens do not trigger immune responses as a whole. Instead, the immune system recognizesspecific partsof the antigen.These specific regions are called:•Antigenic determinants•orepitopesWhat are epitopes made of?•A fewamino acidsin a protein•Or a fewsugar unitsin a polysaccharideEach species has unique proteins and carbohydrates. This chemical uniqueness is why organismsareantigenically differentfrom one another—and why the immune system can tell “self” from “non-self.”1.4Types of AntigensNot all antigens come from outside the body. Based on their source, antigens can be classified intoseveral types:1. Autoantigens•These areself antigens.•They belong to a person’s own body.•Normally, the immune system does not attack them.2. Alloantigens•These are antigens found inother individuals of the same species.•A common example isblood group antigenslike A and B on red blood cells.3. Heterophile Antigens•These areidentical antigens found in different species.•They can cause cross-reactions between unrelated organisms.

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Study Guide1.5Antigens in MicroorganismsA single microorganism, such as a bacterium, can havemany different antigensthat stimulateimmune responses.Common types include:•Somatic antigens–found on the bacterial cell body•Capsular antigens–located in the bacterial capsule•Flagellar antigens (H antigens)–present on flagella•Exotoxins–toxic proteins released by bacteria that are strongly antigenicEach of these components can be recognized by the immune system and contribute to immunity.Key TakeawayAntigens are thesignalsthat alert the immune system to danger.Their structure, location, and type determine how the immune system responds.2.Cells of the Immune SystemThe immune system works closely with thelymphatic systemto protect the body from disease. Itsmain working cells are calledlymphocytes, and they all begin their life in thebone marrow.2.1Origin of Immune CellsAll immune cells develop fromstem cellsin the bone marrow.These stem cells areundifferentiated, meaning they are not specialized at first. Throughout life, theykeep dividing and replacing old immune cells, ensuring the immune system remains active andeffective.2.2B-Lymphocytes and T-LymphocytesThere aretwo main types of lymphocytesinvolved in specific immunity:

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Study GuideB-Lymphocytes (B cells)•B cells are named after thebursa of Fabricius, where they mature in birds.•In humans, this exact organ does not exist.•Instead, B cells are believed to mature in thebone marrow.T-Lymphocytes (T cells)•T cells leave the bone marrow and mature in thethymus gland.•The thymus is located in the neck, near thetrachea and thyroid gland.Both B cells and T cells start in the bone marrow butdevelop in different organsbefore becomingfully functional.2.3Overview of the Human Immune SystemAfter maturation:•B cells and T cells move tolymphoid organssuch as the lymph nodes, spleen, tonsils,adenoids, and other tissues of the lymphatic system.•These cells form the foundation of the body’stwo main immune responses.

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Study GuideFigure 1An overview of the human immune system. T‐lymphocytes and B‐lymphocytesoriginate in the bone marrow and then are processed in different body organs beforeproceeding to the lymph nodes, where they provide the underpinnings of the two types ofimmune responses.2.4Development Timeline of Lymphocytes•The transformation of stem cells into B cells and T cells begins around thefifth month afterfertilization.•A complete set of these immune cells is formeda few months after birth.•After this, lymphocytes continuously circulate and populate lymphoid organs throughout life.

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Study Guide2.5How an Immune Response BeginsAn immune response does not start on its own. It requiresantigen presentation:1.Microorganisms enter the body.2.They arephagocytized(engulfed) by phagocytic cells such asmacrophages.3.The macrophages process the antigens.4.Antigenic determinantsare displayed on the macrophage surface.5.These are presented to specific B cells and T cells.6.This presentation triggers a targeted immune response.2.6Clonal Selection TheoryTheclonal selection theoryexplains how lymphocytes recognize and respond to antigens.Key ideas of the theory:•Each lymphocyte hasspecific receptorson its surface.•These receptors aregenetically determined.•B-cell receptors areantibody molecules.•T-cell receptors are made ofclusters of amino acids.When a lymphocyte encounters a matching antigenic determinant:•The receptor fits the antigen like alock and key.•The lymphocyte becomes activated.•A proper match also occurs betweenMHC moleculesand their receptors.2.7Expansion and Self-ToleranceAccording to the clonal selection theory:•Lymphocytes forall possible antigenic determinants already exist, even before exposureto antigens.•Once activated, a lymphocyte produces many identical cells (aclone) with the samespecificity.•Lymphocytes that could react against the body’s own cells aredestroyed or inactivated.•This prevents harmfulautoimmune reactions.

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Study Guide2.8Two Main Types of Specific ImmunityThe immune system provides specific resistance to disease intwo major ways:1. Antibody-Mediated (Humoral) Immunity•Centered inB-lymphocytes.•B cells differentiate intoplasma cells.•Plasma cells produceantibodiesthat circulate in body fluids.2. Cell-Mediated Immunity•Centered inT-lymphocytes.•T cells directly attack infected or abnormal cells.•This type of immunity is especially important againstviruses, cancer cells, andtransplanted tissues.Key TakeawayThe immune system relies on carefully developed B cells and T cells that originate from bone marrowstem cells. Through antigen presentation and clonal selection, these cells work together to providepowerful, specific, and well-controlled protection against disease.3.Antibody-Mediated (Humoral) Immunity (AMI)Antibody-mediated immunity is one of the body’s main ways of fighting infection. It is carried out byB-lymphocytesand the antibodies they produce. Let’s walk through this process step by step in a clearand student-friendly way.3.1Activation of B-LymphocytesWhenantigenic determinantsenter the body and reach thelymphatic organs(such as lymphnodes and spleen), they activate B-lymphocytes.After activation, B-cells follow two main paths:•Plasma cellsThese are fully differentiated cells thatproduce and release antibodiesspecific to the antigen.

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